http://www.youtube.com/watch?v=wdTSvoRAgp8 Only works in Clockwork Knight Fukubukuro (Japan-only compilation). Code is up up down down left right left right L R L R on the mode select screen, then start a game on Part 2. Credit goes to Yakumo of Assembler for digging it up (apparently?) and Tongara for immediately telling me about it. From what I gather, it was only published once in some magazine. This is pretty fucking big!
I like the choice of music But from what I read on the wiki, he was already playable in the "Bosses Galore" option? So maybe this was part of the original plan. Clockwork Knight 2 is an odd game - I think the first two bosses can be controlled by the second player with the correct codes. I need to play this again some day - I don't even remember the bosses galore mode it's been that long.
Jumped the gun, code also works in the US version, which is a later revision of the game- EU and JP releases seem to be missing the box screen and associated extras (Movies, Bosses Galore, title screen that adds a character each time you complete the game/changes entirely on holidays, and now Ginger!).