After an 8-hour hackathon I'm happy to announce
mobi2lrf. Converts (non DRMed) .mobi/.prc files to LRF. Supports all three levels of compression in .mobi files. Also converts metadata. Part of
libprs500 v0.4.37. It's integrated into the GUI as well as a commandline tool, so using it is just as easy as using the rest of libprs500.
This is an initial release, so it likely is crawling with bugs. Thanks to darkninja for the HUFF/CDIC decompression code.