In this question, people use various ways to notate double sharps:
All of these are reasonable ways to work around the lack of an available double sharp symbol. The same applies to the other accidentals- naturals, flats, etc...
Bret Pimentel has created a jQuery plugin that "attempts to identify accidentals (flats, naturals, sharps, double-flats, and double-sharps) typed in plain text, and convert them to the correct symbols." It's explained and available at his website and the source code is on github.
Could we look into integrating this plugin with this site? Bret might even be interested in helping.