(Weirdly, according to http://SteamDB.info it should be found here: HKEY_LOCAL_MACHINE\SOFTWARE\square soft, inc.\final fantasy vii > AppPath
My version has no such "Square Soft, Inc." key -- I wonder if this is actually the 1998 version.)
It's probably case senstive, but that's the node that WM uses to find the installation of the 1998 version. I seems to work for the recent ones too.
If you reference that programmatically it should end up reading correctly. If you're navigating via regedit you'll have to go through the machine's virtual store then the Wow6432Node. So the actual address is:
HKEY_LOCAL_MACHINE\SOFTWARE\
Wow6432Node\Square Soft, Inc.\Final Fantasy VII\ (64-bit versions of Windows use this node to separate 32-bit applications' entries and Windows is smart enough to add that to the search string when queried.)
That's probably it for the Steam version too. The exe uses that address explicitly so that's what you should use too.