If anyone else wants to try, I attached decrypt.py.
- unzip the python script attached
- put ddojbl.bin from the pgm program files zip into the same directory
- run decrypt.py
This should produce ddojbl_raw.bin and ddojbl_kovplus.bin
If you have MAME and the kovplus roms, please unzip and replace p0600.119 with ddojbl_kovplus.bin. If you run it you should get DoDonPachi DOJ, just with corrupted graphics and weird sound. You can copy over the tile and sprite roms from ddp3 (renaming them to the corresponding files, of course) to fix the graphics.
If that works for you, you can try burning ddojbl_kovplus.bin into an EPROM and see if that runs on the kovplus board... The program ROM should be socketed, so you can just try it and reverse it back later.
If you have a hex editor you can open ddojbl_raw.bin to examine the unencrypted game.
This is probably all I can do without some kind of logic analyser.