I currently use hypermail for archiving two lists only because I like the
little formatting hypermail does. In most cases, 95%, just adding the
italic font and because the rest of the stuff is in proportionally
spaced font makes the text far easier to read, IMHO.
Have you looked at the options for the text/plain filter? The
text/plain filter provided with (later versions) of MHonArc has various
options for processing the text, including leaving out the PRE element
to allow regular (default) typeface to be used in rendering.
The MIMEFILTERS resource page describes all the default filters
provided by mhonarc and what options they support.