The only thing I have seen is an SD3 three player hack, haven't seen anything else for any other games.
Something I found interesting is that the American SwoM box mentions nothing but "GBA Link Cable", but the Japanese box I just got mentions 1-2 players simultaneously.
I think if it actually had 2 players we'd have heard something about it, but I can't help but think it's weird they put that on the Japanese one?
"Dude, you're in college. In my mind's eye, you're still, like, 12 years old and humming songs from SoM. My poor brain is trying to catch up." ~ My cousin Jamy