I just finished implementing support for man pages in the help plugin, and it seems to be working fine so far.
Configuration looks like this:
And man page like this:
You can also specify more than one path for man pages (e.g.
man:/usr/share/man;/usr/local/share/man;/another/dir/with/man/pages) if you want/need to.
It searches for man pages recursively (be careful with this), and it handles plain text, gz and bz2.
I still need to test it on Windows, update the project files and commit the changes (BerliOS is down ATM). Right now, it's only configured to be compiled using autotools.
I hope you find it useful (when I be able to commit it).