I'm proud to announce the first homebrew game for the SPMP305x! It's nothing special, just a pong-clone, but it comes with full source. Download it at
http://spmp305x.spritesserver.nl/mp4-pong/ ; the .zip contains the compiled binary, the .tgz has all the source-files licensed under the GNU GPL v3.
If you want to help to decypher the secrets of this chip, please go to
http://spmp305x.spritesserver.nl/ and add where you can, or subscribe to the mailing list. Together, we should be able to get the amount of unemulated games available for this chip well above the 20-ish we have now.