It is definitely possible to use Avast without Amavis. With Postfix you can use for example the “After-Queue” filters .
In the filter script simply run the “scan” command on the mail content file.

We do however only support Debian and RHEL/CentOS systems, so even thought Avast may work on Gentoo at the moment
(we did not test it), no one can guarantee, that it will not break with the next VPS.