Funny you mention making a RAM expansion... I had started doing something very similar too. There were two ways I was gonna go... make my own RAM cartridge, or something that plugs into the cart signals internally. I liked the idea of making my own carts because the prices for the original ones are crazy!
Do you want hard copies on ROM or do you just need the *.BIN files?
Hopefully soon I'm going to attempt an actual rewrite & expansion of the OS, not just using the empty space that's there already.