I own an MU90R, great synth, beats the crap out of anything Roland ever made back then anyway.
I can't answer your USB question, but I do have both bibles that came with the thing and Yamaha include a diagram for the serial port for which I also seem to think I have a drive. I could look for the manual and try to scan the pinout if you feel like making your own cable and trying the Yamaha drivers. This will not work for DOS games though and I am unclear how it might affect Windows applications that aren't for music production.
Really your best option is to find a real MIDI cable which connects to the game port. I need some too and last i looked they were expensive and of lower quality than my old ones. Hong Kong seems to have a stock of them now though, but I can't guarantee their reliability or if they even employ the opto-couplers they are meant to have - see eBay - found by typing "PC MIDI Cable 15" into the search, set to Worldwide for better results. I don't know where you are, but you might have better luck than me, the UK seems pretty techno-phobic versus the USA and most of Asia
Quick thought: Check the MIDI tab in the Multimedia applet in the Control Panel. It might actually show up there (The USB MIDI adapter thing) but I have no idea how well it will work if it does.
Also, Yamaha have the manual online, go to Page 44 (As the browser sees it, not as marked in the manual) for pinouts; http://download.yamaha.com/api/asset/file/?la … m&asset_id=2559