Awesome. After posting this thread I started browsing around for a Panasonic Q. Then I saw that I would have to use a (fairly ugly) Q specific gameboy player that would cost approx $200. Regular cube it is.
FWIW, I can confirm that the Japanese 'Cube/Game Boy Player can play US GBA titles. I own one a Panasonic Q with one of those overpriced Game Boy Players. Thankfully, the unit uses the same Component Cables and Broadband Adapter though.
Unless you're super-set on getting a new GameCube, I'd suggest picking up a used US one as they're super-cheap (I recently picked one up at a thrift store for $5). Then, of course, all you need is a freeloader for your Japanese titles...
"The ultimate weakness of violence is that it is a descending spiral, begetting the very thing it seeks to destroy, instead of diminishing evil, it multiplies it."