In spamassassin's configuration I use add_header to put in some extra header lines.  For example, one I like is:
add_header all Scores TESTSSCORES
which adds a header something like "X-Spam-Scores: BAYES_00=-4.9,NO_REAL_NAME=0.16", which is useful to see how an email that has not been marked as spam has been scored.

Unfortunately spamass_milter only extracts specific named headers (Report, Flags, Status) using replace_or_insert, so the custom header is lost.

It would be good to have an option to allow wildcard headers through, specifically "X-Spam-*".  Or to just use the complete message as returned from spamassassin.

I am using Gentoo Linux,x86, spamass_milter 0.2, sendmail 8.12.10, spamassassin 2.63