The thing is, the config page of MediaPortal is basically a command line builder. If I run the same thing as I apparently select in the configuration as a command line (to DTAgent.exe) it works. So that leads me to think that something has changed in the structure of the command line arguments that MediaPortal is just not picking up.
I've taken a copy of the source code for MePo, and am going to try and work out what it's doing & then hopefully fix it. It's probably not a high priority for the devs, but it is for me, since I've paid for Daemon Tools Pro and don't really want to use Lite (and VCD doesn't like my HTPC any more).