I just found a guide for how to connect to other people and play this game cooperatively online. It is amazingly complicated and unintuitive, just like everything else in SMBX.
I just have to know, has anyone tried or done this? I'm really interested to know if it works. If anyone manages to get this to work, post here and let us know about it! I think it would be really cool (and likely game-breaking!) for people to be able to play Moon Base together.
I played online with some friends on the SMBX forums before they disappeared off the face of the earth. It's not too hard, as long as you can find someone who knows how to 'forward posts' (Whatever that means)