I was going to ask, can we use the existing cc65, or do we need a special build?
I've managed to get my MEGA65 to boot, though I couldn't figure out how to mount disk images, but didn't really have the time to dig in. I did notice that the keyboard entry seemed a bit fickle. Some of my keypresses didn't register.
Anyway, back on topic, I'm currently writing a game for the C64, so have been thinking about writing something for the MEGA65, and that brought me to the toolchain to use