Nevermind - it was a WAY stupid mistake I made myself. 

The fix I described actually does the job - however, I had used the binaries
of the pkg in mailer.conf, while still starting the old daemons from 
/usr/libexec (and also having that in main.cf). 

Changing mailer.conf back to that path too makes all work smoothly again.

Thanks for listening anyway :)

Jan