I can understand a one-off or a few units for people or whatever, but honesty thinking about it, it would need to be reworked so that there is 1 microcontroller working with midi, redo the screen pcb and possibly the screen itself into a new physical format so it's not terribly hacky, the cart slots would also HAVE to be accessible- They do get bumped, they will crash, they could be wiped, without easy physical access I can foresee many issues. as well all buttons would need to be accessible to adress all of the functionality.
For a "Product" and not just a few custom units, I would suggest ditching the idea entirely and just going with FPGA and r2r DAC or something. Which is a larger scope project but much easier to produce.
As far as mGB and Arduinoboy are concerned the licensing allows distribution with commercial products as long as any modified source is released to the public. But then again, there have been countless man-hours spent on those projects FOR THE COMMUNITY and not for someone to turn around and make a buck on. I don't want money, it was made for it's purpose and will still be maintained that way.