Preprocessing message contents before displaying

[Sean McAfee]

I've been mildly irritated for a while now that whenever a co-worker
sends me an HTML e-mail into which he's pasted some program code, the
code is rendered with a blank line between each line.  I finally dug
into the raw content of one such message, and discovered that each line
of code is wrapped in its own <p> tag.  ಠ_ಠ

Presumably there are some style rules somewhere that call for minimal
bottom margins when the text is rendered graphically, but all of the
text-based renderers I tried showed the code as double-spaced.

So, what I'd like to do is tell Gnus that messages from this co-worker
should be passed through a custom filter before being rendered, so I can
remove all these confounding <p> tags and use <br> tags to break lines
instead.  The content is encoded as quoted-printable, so ideally I'd
hook into the rendering process after the content is decoded, but if
necessary I could take the raw content, decode it, tweak it, and
re-encode it.

Can this be done?