Is there a ruby mail-to-HTML converter similar to the perl MHonArc?
Wow, I remember using MHonarc back in 1995. If it does exactly what you want, why not use it? You can call it from a Ruby app with backticks.
If you're really looking for a Ruby solution, the description of the Ruby-Talk mailing list says it is powered by "Shin-ichiro Hara's Ruby smart archiver" so you might want to look for that.
If I were to build an archiver today, I'd probably set up a Rails app that stores incoming mail in a database (or better yet, a Solr index) to provide searchability as well as browseability. The capability to receive and/or process email is built into ActionMailer. (Outside of Rails, I'd use the Mail gem).