While I would like to see something similar for other reasons, it would not be possible to use it for the purpose you describe (nor would it make any sense). The reason is that OPL chips are not General MIDI synthesizers (in fact, they don't speak MIDI at all for that matter), and at best would sound pretty poor if you were able to rig something like this up. Games that have native support for OPL music program and drive the chip directly with sounds and commands that it understands and that the game developers have tuned to get the most out of the chip. Using a generic set of instrument data that has been mangled to be GM-ish is not worth the effort (trust me - I've done it on real OPL hardware back in the days of Windows 3.1, when all game music was MIDI and there were no wavetable synths).

A much better solution would be to either get a sound card with better MIDI support (e.g. soundfonts) or an external MIDI synth. Or as a last resort, there are some software synths available such as SynthFont (a pain to hook up as a realtime soundfont synth), Roland Virtual Sound Canvas (actually not too bad), and Yamaha's XG softsynth (not all that great in my opinion). I'm sure there are others.

You can also replace the GM.DLS file that the Windows synth uses with any old .dls file, but there aren't as many of them as there are .sf2 (soundfont) files...

I know that OPL chips are not, by default, MIDI compatible, but with appropriate software they have been used for MIDI output in the past, and I for one miss that, which is why I'm looking in the first place.

Is there anywhere I could get some information on writing a Windows MIDI driver?