i got it working by running steam as admin and then adding the 7th heaven installation of FF7 as a custom game. not sure if it'll work with steam link though, i was just trying to get the overlay working so i can use my ps4 controller without remapping a bunch of things. here's the steps:
1. change steam and 7th heaven to run as admin (right click -> properties -> compatibility tab -> enable "run this program as admin")
2. launch steam and add 7th heaven and ff7 (7th heaven version) as custom games
3. launch 7th heaven thru steam (there shouldn't be a UAC popup anymore)
4. launch ff7 thru 7th heaven