Thanks Peter,
As you see, the dropdown list is just a predefined collection of ranges which make easier the inspection of usual Unicode sets. The "javascripter" is highly requested to customize these sets for his own convenience. Alternately, it's possible to stipulate any range directly from the Unicode field, using comma separated values and/or dashes for Unicode segments.
You have a point : the glyphs/ranges available in the current font are certainly documented in the font file itself. The problem is that I don't know how to extract and parse this information! For the moment, the glyph-checking process is "empirical", thanks to the Dave Saunder's outline-trick:
http://jsid.blogspot.com/2006/03/what-and-wheres-that-glyph.html
[Precision: SpecialCharacters names and id's exposed from the script are also hard-coded, despite of your great method using the array SpecialCharacters.reflect.properties. The point is that I wanted InDiCode to be InDesign CS-compatible.]
I rely on users feedbacks and requests to improve and extend the process in the future. InDiCode has been primarly designed as a library and the script hosts a lot of "sleeping" features (exporting methods, UTF8 translation...).