I managed to add General MIDI support to V5! I just took the GMIDI.IMS from the Sam & Max WIP demo. Thanks to the redundancy of ROLAND.IMS and ROL_330.IMS, I don't even have to sacrifice anything. I hex edited the exe to refer GMIDI.IMS instead of ROLAND.IMS, and it works flawless on my SoundCanvas.
So adding General MIDI support to V5 games is very possible.