I've actually tried the modified ROM before with channels 11,12,13,14 midi channels, but it didn't work by itself (without modifying ArduinoBoys settings) so I've tried also combination with ArudionoBoy settings to 11,12,13,14 with stock ROM (1,2,3,4 channels) and it does work, so I'm quite confused about the ROM settings, if it is really necessary. But as I wrote earlier when I set ArduinoBoy to e.g. 1,1,2,3 (4 for poly) and send note to midi channel 1, it will only play one pulse channel..
The RK-002 would be definitely an option, thanks for advising it --- but it is little bit expensive and it making things even more complicated, so in case of any sudden bug, or weird behaviour of my setup that would be another thing to check when finding the root cause. So I rather choose the way to make it work, because I think it is somehow possible.
I've also tried to contact trash80 about this, but he didn't respond to my e-mail. :<
Maybe I'll try to ask my friend, which does know C language quite well, so maybe he would be able to rewrite the code for my needs.