Question : if I write any more Rcas2 games, to make them compatible with this what memory can I use, beyond the obvious (400-7ff) ? Is it just c00-Fff ?
Yes - I am only decoding the lower 4 address lines so the cart only 'knows' A0-A11, which gives it 1K addressable space... $800 to $BFF is used by the different consoles, so that leaves the addresses you quote. I guess that adding a second latch is possible, which would give 64K to play with, but there's no way to add RAM to the cart, as the MWR signal isn't there...
Not sure you saw my message some time ago: on real hardware, using long branches causes timing issues for the 1861, because it loses sync due to the three CPU cycles these instructions use. I've managed to patch Pong/Hockey and Combat, so they play fine. Invaders is giving me more problems, as it has a lot of long branches and 2 really long routines as far as I can see that go beyond the 256-byte pages. At the end of every cycle, the screen flickers as a result... Since Emma doesn't show the problem, testing can only be done by burning new versions to EEPROMs, which is rahter cumbersome.
As for ideas for new games: I was going to try and see whether I could adapt your Scramble code into a Flappy Bird clone - haven't gotten around to it yet, so I hope you can beat me to it!
There's a complementary cart reserved for you - if you PM me your details, I'll send it out with the rest of them...