Damn, in the time it took me to set up an account here someone else posted a nice nugget of reverse-engineering-info here
And he's Dutch too :P
I myself have made some progress on reverse engineering the .bin-format game-files. They're basically 128 byte headers, with the arm machine code to be loaded at adress 0x24C00000 directly after it. The machine code can then use a standardized OS function table to let the OS do stuff like play midi, display gfx, etc.
I have just finished setting up a wiki and a mailing list for more spmp305x-reverse-engineering. They can be found at
http://spmp305x.spritesserver.nl/ . While my main use was to dump info for game-files inthere, I can't see why delicious info like io-mappings can't go in there too, so you're welcome to edit it in.
And to show what I've done up till now: You can download a small demo here:
http://spmp305x.spritesserver.nl/demo/demo-0.1.bin
It doesn't do that much (play a midi until a key is pressed) but it's enough to show there's potential, I hope.
Erant: I've been wondering over the display subsystem too. I haven't actually seen any framebuffer-like bit of memory containing the current image in main RAM. Perhaps there's a second bit of memory somewhere inside the sunplus chip?