I've written a little spamassassin filter in perl. The only requirements
you may not have installed is AnyEvent. Its pretty common so your os
most likely has it packaged. If not:
# cpan -i AnyEvent
Obviously you will also need the spamassassin daemon running.
I've been using it for about 3 days so far without issues, though there
may still be bugs lurking. One is likely to be the need for a message
size limit. Not sure what that should be though.