You would likely have to pose that question to rottendog directly:

http://rottendog.us/contact%20us.html

Rdog probably uses a large eprom and a dip switches to pick what ROM set gets put in the address space the CPU is looking for. Take that large ROM and inject your special game code you want in the spot you want to use.

Only issue is that this Rottendog board uses custom software, so they have somehow modified the stock roms to work on their board. Because of this, I don't think directly injecting the home rom to the correct location would work.

This is an old thread - but I too am interested in the progress of this if anyone has any insight.
If someone was good at searching through a dump of the code, they should be able to mark out the 'sections' of code in the RD custom rom and compare the code to the original ROM to ensure you are in the right place. If I had more time in my life - I would consider doing it. I could assign this job to one of my students .... humm .... learning experience for sure.
-Rob

I too would love to see this.

Yeah, It would be awesome if there were some added features for Gorgar. Like shoot drops left to right to increase bonus multiplier, or start a spinner hurry up.

Donations ready.