It may be of some interest to forum members to know that Asymptote open-source project has some code to produce PRC 3D models in PDF. Part of it can be used as a standalone rudimentary PRC producing library.
Just an update - the LGPL C++ code for PRC output is live and kicking.
Its different revisions can be found in Asymptote, MathGL and s2plot.
It remains rudimentary due to PRC format being rich in features and poor in docs.
It will remain so since I do not see much future for 3D PDF (and Adobe concurs), but in the meantime,
before 3D WWW finally delivers, it may have some use.
If anyone stumbles upon this post and gets interested - contact me for details.