I gots a faster, better, (and more importantly), a relatively free idea that should work better than the S-YXG70.
The web site of this guy known on the VGMusic Forums as "Tony Thai" has the
Full (non shareware) version available of the S-YXG100 and S-YXG50 available for download in a small corner of his site.
The site's main function is NOT warez, as far as I know, he mainly has his latest MIDI and mp3 compositions there, most of the MIDI's are XG, hence he decided to upload the install program for it.
So, to keep it a little general, I'll link to the main page of the site, which does not have direct links to the program or any other programs for that matter, so I don't know if that is still considered a "Warez" link.
Tony Thai's page is
here. Last I checked, the "miscellaneous" link on his page should lead you to the page containing the link to the download.
I hope I'm not crossing the line here, though.
And, in regards to the funtionality of the S-YXG100 in FF7, I've done tests about half a year ago with the Shareware version of it in-game, the only real differences is that it uses the XG MIDI data setting instead of the General MIDI data setting in the config.