'If thee champion has at least one AP scaling ability, thee can be built AP' - Unknown

Yeah, I pretty much made that up. Someone might have actually said it, but non the less the point stands. Considering that Pantheon has not 1, BUT 2 AP scaling abilities (both @100% from AP), we can safely assume that he can be built as an AP champion.

Obviously, this is waaaay off-meta, boarder-line troll build. However, it is quite FUN and QUITE VIABLE - viable mostly because you will be filling the support role.

THE PHILOSOPHY OF AP PANTHEON

1) MAN LEAP a.k.a. Grand Skyfall into battle.
2) All affected targets take massive damage.
3) Focus one by using Aegis of Zeonia.
4) Spear Shot if the target lives.
5) Attack and Heartseeker Strike if the target still lives.
6) OPTIONAL: Target escapes or dies. You can't do much either way. Suck it up and find another target.

The idea is to NUKE/BURST/OBLITERATE/DESTROY your target as quickly as possible. Once both Aegis of Zeonia and especially Grand Skyfall are on cooldown, you will have to back off a bit and harass with Spear Shot (since you won't build any AD, this will become more of a 'Toothpick Shot' as the game goes on).

To sum it up:
1) Burst when you can.
2) Harass, CC, poke, until you can burst again.

MAGE

This rune build focuses on sustained damage over time.
Arcane Comet - provides additional damage WHEN it hits. If you time it with Aegis of Zeonia it will land 10/10 times. Not as reliable with Spear Shot, but still it can be used as a potent harassment tool. Spear Shot can also be used to reduce comet's cooldown (20% of CD is reduced for every single target ability which lands on the target). Make sure it is ready if you are going for the kill.
Manaflow Band - great for Mana sustain, especially if you plan to poke a lot.
Transcendence - helps reach CDR cap and enables that the extra CDR isn't wasted, but transformed into AD/AP instead. Mostly chosen for Grand Skyfall.
Gathering Storm - makes sure that you are increasing in power as the game goes on.
Sudden Impact - both of Pantheon's initiation tools, Aegis of Zeonia and Grand Skyfall, trigger this effect so you will always deal some extra damage whenever you engage.
Ultimate Hunter - additional CDR for the MAN LEAP. Grand Skyfall deals massive damage, but landing it isn't always guaranteed. This rune will increase the frequency of MAN LEAPS, therefore increasing the chance of landing a successful one.

JUGGERNAUT

This rune build focuses on sustained health over time.
Aftershock - great for any initiator, especially for this build that lacks heavy armor and MR items. Damage from Aftershock shouldn't be troublesome to land. Even if you don't, armor and MR gained is the main focus.
Font of Life - good thing here is (same with the aftershock) that this rune will activate with both Aegis of Zeonia and Grand Skyfall. Unlike Aftershock, Font of Life has no cooldown, so whenever you engage with aegis, the target will be marked for your carry to heal.
Second Wind - this is Pantheon's personal Font of Life.
Overgrowth - makes you tankier. Getting tankier is good.
Sudden Impact - see above.
Ravenous Hunter - Combined with Second Wind, this rune provides a complete healing effect: you get healed when attacked and when you attack (applies to spells only).

ASSASSIN

This rune build focuses on dealing burst damage.
Electrocute - procs when you do the basic aegis >> attack >> spear combo, adding to the burst.
Sudden Impact - see above.
Eyeball Collection - this rune needs a little time investment in order to shine. Once the stacks are completed, it provides a nice amount of additional damage.
Ultimate Hunter - see above.
Coup de Grace - with Heartseeker Strike's passive, Pantheon is already a fearless executer. This rune will further increase that. Also don't forget its adaptive bonus.
Presence of Mind - although Mana restoration is great, the real reason why this rune is taken is because of its CDR effect on the ultimate. Again, we want that MAN LEAP ready as often as possible.

DEFENDER = Exhaust + Flash
I would run this 48% of the time. This is a safe choice of summoner spells. Exhaust is an excellent spell for running down and defending yourself/your carry.

AGGRESSOR = Ignite + Flash
If I'm not running DEFENDER, than I'd definitely go for this choice (again around 48% of the time). Ignite provides bonus damage and a good way to secure takedowns, especially for a melee champion. Its grievous wounds effect is highly appreciated as well.

CHASER = Ghost + Flash
If I feel like it I'd go for these spells (~1% of the time). These provide excellent mobility, which enables you to run down your opponents and make sure they don't escape. Also, allows you to escape, since Pantheon does't really have escape mechanics, other than some cleaver use of Aegis of Zeonia and, perhaps, Grand Skyfall.

ROAMER = Teleport + Flash
Similarly, this is a rare occasions choice (~1%). Teleport grants more freedom over MAN LEAP and opens greater opportunities for ganks. You can gank wherever you want and quickly return to your carry.

Note: I had to classify summoner spells somehow, but that doesn't mean you have to be aggressive if you chose Ignite, or that you have to gank other lanes whenever your Teleport is off cooldown. These are all just options. You choose what fits your needs.

Considering that you are going to be your teams support , a blend of buff/debuff items alongside AP items is a way to go. Here we will cover 4 item groups: GOLD INCOME ITEMS, BOOTS, UTILITY ITEMS, POWERFUL AP ITEMS.

GOLD INCOME ITEMS
Eye of the Aspect - get this if you need to be tankier and if you prefer Relic Shield support playstyle.
Eye of the Watchers - get this if you want to poke/be more aggressive and if you prefer Spellthief's Edge support playstyle.

I wouldn't consider going for the coin with this build. Pantheon's ability to support comes from his aggressiveness and both Relic Shield and Spellthief's Edge do a better job than the Ancient Coin does.

BOOTS
Pretty much all boots choices are viable (apart from Berserker's Greaves - they suck for this particular build). However, I will narrow it down to 3 main and 1 situational choice. Main choices include: Ionian Boots of Lucidity, Ninja Tabi and Mercury's Treads. Situational choice includes Sorcerer's Shoes.

Sorcerer's Shoes are situational because you might find them effective if the enemy team is REALLY heavy on magic resistance. Other than that, they are not worth it. They cost 1100 gold, which is equal to the cost of tabi's and merc's and 200 gold more expansive than ionian's, yet they provide only spell penetration with the move speed increase. All 3 main choices provide 2 additional effects alongside movement, making them more efficient.

UTILITY
This category includes items that supporters usually get to buff their team or debuff the enemy team, as well as some personal benefits. Twin Shadows and Hextech Rocketbelt provide movement, damage and CDR. Shurelya's Battlesong and Turbo Chemtank are good examples as well.
Get what you need, but make sure you get it! Don't go overboard, ideally ONE is enough so the remaining 3 slots can be filled with more AP oriented items. Again, your strength comes from those items.

POWERFUL AP ITEMS
Keep 2 things in mind when you buy some of these items:
1) You want BURST. Grab items that provide you with a lot of AP and have burst effects (example: go for Lich Bane - not for Liandry's Torment);
2) Rabadon's Deathcap IS A MUST.