PG is starting to post EPUB format, which with Calibre command-line tools can be quickly turned into just about any format you like -- I've started posting them in Mobi format to
http://freekindlebooks.org/MobiMirror. These postings contain the PG "legalese" ie they ARE the PG files "file format converted to run on your computer." As such you can do anything you might want to do with them that you can do with any other PG file. EPUB is probably a better place to start from though, not MOBI.