Spamassassin 3.1.2 has introduced support for checking DKIM using the CPAN Mail::DKIM module. Could Mail::DKIM, and its dependencies, be added to the portage tree and a USE flag be added to optionally add Mail::DKIM as an RDEPEND to mail-filter/spamassassin ebuild. I have installed Mail::DKIM using g-cpan and it seems to work OK with spamassassin 3.1.2.
package dev-perl/Mail-DKIM now in cvs
..long time ago.