why wouldn't they work? Japanese GBA games work on North American GBA's. But I can't say about the N64, never tried it. I'll ask my kids though, they will know.
I have been told by a close friend that you can modify the cartridges to work in an American N64. It has something to do with filing down part of the plastic on the inside of the cartridge...where it plugs into the N64 bus. He swears that it works on his system...I haven't tried it myself though!
We had this conversation while in Toys-R-Us today discussing whether the Japanese DS games would work in a US DS Lite and vice versa.
DS games are regionfree ;-) I have a lot of games from Japan on it Most of the time the Japanese shopkeepers don't even know this at all...so much for being up to date