The learning curve for Lilypond is really no worse than for any other text-based music typesetting software -- and far easier than, say, the TeX music packages. ABC is a little more simple on the surface, but is so much less flexible as to make it useless for complicated music. MusicXML seems to be a good transport format, but as parent points out, it's clunky: like any other XML format, everything is perfectly readable and will take a year and a day to type.
Lilypond, which seems to me to be the best option for open-source notation software, is probably not a worthwhile system for creating on-the-fly snippets for web pages. Something JavaScript based would be nice for that, and it seems likely that there could be wiki plugins that could be used to create musical examples without huge dependencies (much less specific versions of any given large software packages), or a knowledge of how to use these large, complicated notation software packages. There might still be the problem of being limited to simple examples, but do we really need all of Lilypond and its dependencies to create an example of a Phrygian scale?
...wait, they'll give handj-- I mean, I can go to Starbucks on these asteroids? Sign me up!
"When the going gets tough, the tough get empirical." -- Jon Carroll