Lilypond comes bundled with a couple converter scripts. We already use the ABC converter. Both supported formats are however less common than MusicXML, which is used as an interchange format throughout different notation programs. Using the bundled XML->Lilypond converter, we would open up importing/exporting music from existing GUIs onto Wikipedia, potentially reducing the effort it takes to write LY.
However, MusicXML files are very hefty, and would not be able to be modified as efficiently on MediaWiki without copying over the notation, importing it, modifiying it, exporting it back, and pasting here, which would be more complicated than our current state.
Should MusicXML in <score> tags be supported?