So I played a bit around with the Steam files with the following result:
Steam do only look if there is a program in the games folder which is called FF7_Launcher.exe, it doesn't matter what program it is.
The launcher itself do only look if there is a program called FF7_en.exe (depending on the language you use it may named differently). In both case the Steam overlay do work, but I guess achievements will not.
As far as I understand it they are handled by the launcher and the save files in the user's document folder. Is it possible to force the old FF7.exe to use the rerelease save file system? Maybe with a small program which syncs both save folders, the old and the new one, with each other.
Even if this wouldn't work to enable achievements directly it could be used to get them by loading the save files with the rereleased FF_en.exe.
I haven't tested how exactly 7th Heaven do work with that, but if everything would work together I could free up some space on my HD drive by deleting one FF7 installment. It also may convince some diehard Steam user to convert the game, but still be able to use the overlay of steam.
