It seems to me that the best thing we could to is to focus on getting the Perl::Milter implementation up and running because that would possibly remove a great deal of overhead.
It looks like the Perl::Milter module required multithreaded perl -- we've got that on the test machine, but not the prod machine.
Also, it looks like you can't install Perl::Milter using MCPAN, because the Makefile.PL script requires access to sendmail libs (maybe source, I can't remember) -- that's where I left off when I was last working on it.