One way to make this easy and a priori trouble free is to modify the PortGroup command such files in $portdir or $filespath have priority over those int the normal PG directory (and don't advertise the new feature widely).
In fact I should look at how PGs are loaded and if they behave as expected when called from inside a pre- block. I can't recall I ever tried that and off the top of my head I think they might run into the same problem I'm seeing. R.
