Starting with 2.6.4, there is an SNMP AgentX subagent which ships with the package. This subagent should be installed if USE=snmp... Since the lack of this support means that the package was not examined by the maintainers when it was bumped, I've marked it as QA, rather than enhancement. I have a work-in-progress ebuild in my overlay[1] which adds this support. I have not yet modified the init script to start the daemon, or added conf.d support for enabling said daemon, but it's coming as I have time (possibly using the changes from bug #263180).
Added with amavisd-new-2.7.0. Closing.