FWIW, we had a similar problem where we needed to point to an external graphics library (not part of the project root). We didn't want authors to have to deal with the location of the graphics (which might change anyway).
Our solution is to pass in a 'graphics.dir' param from the command line. I modified mediaobject.filename a little different from Jirka's suggestion: it prepends the $graphics.dir to $filename for the generated HTML as well as the hhp file enumeration. Seems to work okay - the author specifies only the file name (no path info) when referencing images, and we can locate the library anywhere at build time. Denis
