There is a patch available that makes clamassassin using maildrop instead of procmail. As far as I've seen there are 'procmail' and 'maildrop' keywords, so can anyone enhance the ebuild with a switch to en-/disable this patch and the related dependency?
It seems as though the upstream author is preparing to include this change in the next release, which is due soon. I'm going to mark this LATER -- if there's no new release in the next couple of weeks, I'll work something out. Hopefully I'll have a bit more freetime by then as well.