I think I found the coin counter for SonSon. The game allways writes 0x01 to 0x301F when inserting coins into coin A and 0x01 to 0x301E when inserting coins into coin B. Is there a better place to discuss these things (I found a couple of things while making my GBA emulators) or are there some MAME devs around here who can make the changes to the MAME source?