NeoLemmix supports this ... and I would be extremely surprised if anything's changed in this regard in SuperLemmix.
Thanks namida, I can confirm that SuperLemmix indeed hasn't changed the way the engine handles music at all.
So yes, it seems that if you have Audacity it's already possible to create .ogg music files that are capable of looping to/from a particular position, which enables them to have an "intro" portion, then a looped portion. Such files will also play as intended in others' copy of the game as well.