I haven't tried this, but looks like there a few Java APIs out there that can do this; if you can do it in Java, then you can generally do it in PRPC as well.

See : http://stackoverflow.com/questions/2091803/how-to-convert-html-to-rtf-in-java

It mentions 'iText' in there as an option - which is interesting - because PRPC ships with iText installed (albeit it an older version that the current version) , which might mean you can just go ahead an use it - without having to import a third-party JAR.