I have found that mapping my usb joypad for DGen games requires me to edit the dgenrc file inside the hidden .dgen folder. I have all the buttons working properly except for being able to exit out of roms. It looks like mapping joypad buttons can only be done to valid Genesis controller buttons (a, b, c, x, y, z, mode, and start). It has a key_quit command which is mapped to the escape key. I haven't been able to find a way to map the key_quit to a joypad button. I have been trying different things out and searched forums high and low with no success. If anyone has any insight I would greatly appreciate it!
This link goes to the main dgen page which shows how the file works.
http://dgen.sourceforge.net/