Pipeline, could you please add the mp_ui.desktop without the MimeType line to your .deb? I'm not as sure about the defaults.list file; I think it would be a good idea, but there may be side effects...

If those two files are updated with the .deb, then the user doesn't have to do anything except change the two config files in the home dir.

The config files should probably have an example of a file type that is associated with the built-in Media Player, just so it is clear how to associate files to a hildonized, non-CLI app via dbus-switchboard.

Please post your results here. I'm curious to find out! Could you link to the thread where you discuss this custom script?