The reason I ask is that it would be really nifty to be able to share or export these tunes. Either via MIDI, or preferably, via another notation format. If they will be fairly simple melodies, ABC might be a great output format, which converts easily into sheet music notation and midi.
Otherwise, you might want something fancy-pants like Sibelius (or whatever people are using nowadays for notation.)
They've both been around for awhile, work from custom text-formats, and support a ton of music typesetting features.
Import/Export is on the TODO list. If there ended up being a custom RiffUp format, it would be open and documented.
An open format for export would be great!
Disclaimer: I have no experience with MusicXML. The vast majority of the music I want to notate fits on a single staff, and ABC is brilliant for that -- it's a well-established portable open music format that's easily entered and edited by hand. (Technically speaking it can do multi-staff music as well, but it starts to get a bit over-complicated and hacky at that point, IMO.) http://en.wikipedia.org/wiki/ABC_notation
PS The "Score" example on the ABC notation page is terrible, but it's a limitation of the lousy standard abc2ly translator, not the ABC format. I've got my own abc2ly which does a much better job of it...