My Movies ExternalPlayers.xml
My Movies has an xml file where you can define external players (for File Structure Playback ONLY) and how to call them. However, it doesn't appear that DVDFab's Media Player puts in the "typical" registry keys during its installation to be able to easily manually add it into the list of players there. In the HKey-Local Machine there should be something like a SOFTWARE\DVDFab Media Player\InstallPath registry key entry with the install path. You might be able to manually enter a hardcoded path in My Movies' xml file, though - I haven't tried that yet.
Unfortunately, Media Center doesn't allow overrides for .ISO file playback and it only plays through it's mechanism - DVDFab would have to write a player plug-in for Media Center to support .ISO playback. This is what TMT and PowerDVD have done.
At least that is what I understand how it works, anyway.