jimLee:
I certainly hope you guys are making some progress on your projects for next time.
Ha! Well... Sort of. That is, if "madly acquiring parts" counts as "some progress." 
I've decided that my next adventure is going to be replicating--and then enhancing--the functionality of my breadboarded rat's nest on actual Capcom arcade hardware. I believe I mentioned that I'd found some games that used the synth as well as the PCM chip (which I don't have) for music? Well, as I've continued to rip music, I've found more and more of those--probably half of the Capcom CPS1 library! Poo!! I wasn't prepared for that at all. The PCM chip on those boards is an odd beast, with its own private bus and a wacky 4-bit data format.
So, I had this funny idea a while back: What about just taking the actual arcade motherboard, piggybacking the Z80 audio CPU with the Arduino, and bus-mastering the Z80 bus to take over control of all the audio hardware. I've spent about a week's worth of nights studying the hardware, and I believe I have a workable plan. And it'll be cool! Best of all, I think I can do it with only a few minimal modifications to the motherboard and without removing any components--which seemed like the only polite thing to do, as these boards are sort of collectible anymore.
So I ordered a Street Fighter II board off eBay (it's a CPS1 title) and should have it next week. It's remotely possible that I might have something working with it by the next time we catch up, but at the very least I'll bring the disassembled boardset along and can describe my ridiculous plan of attack.