So normally the way you'd invoke qmail-scanner is to load it with an environmental variable in daemontools. Since this is xinetd, that doesnt work (or didnt, honestly I last did research on this about 8 years ago) so what happens is:
1) the internal qmail-queue binary is backed up (take a look at %post)
2) a new one that does support handing off to qmail-scanner is installed in its place
3) qmail-scanner knows to use the backed up file from step 1
When I first started trying to do something like q-s with postfix I got pretty far down the path of using the handlers system to work with it. That would have allowed a common qmail-scanner pacakge to work across any MTA used in plesk.
What I ran into, and Im sure its solvable, is that the format of data passed from the handler was not compatible with what q-s expected to see from an alternate queue. While poking around for solutions, I ran across someone who had written a policyd wrapper for plesk that would work with qmail. I think this might be the project:http://blog.schalanda.name/archives/171 ... mail.html/