Its a good thing I asked then! Its not a problem, its your project and I understand wanting some control over it. I have a cnc milling machine so I can make my own PCBs (not exactly fab house grade boards, but its fine for my own use or prototyping). I was basically going to build exactly what you've got there in the pic, barebone arduino board with an optoisolator on the serial in. I can understand exactly why you went the teensy/usb route, most people are laptop musicians and want as few cables as possible so having power and serial down a single usb is much tidier.
But if you were to do an optional extra board, with standard midi din in I'm pretty sure you would get some interest. The question is whether there would be enough interest for a fab house run, which was why I was going to go the modding route. Its not a deal clincher, I can go with my web book if I'm using the megadrive but its definitely something I'm interested in in the future, and sounds like e.s.c. and eptheca would be too.