If you are good at the champ you can play it in more than the intended role, I would suggest mid for the roams top and bot. you can roam cleverly avoiding vision going across walls.
There are some top lane Rek'Sai players and at least one high elo support player. There aren't as many resources and players to watch for playing her in those roles, so you might have to figure some stuff out for yourself. It's doable though.