Currently xfce4-pulseaudio-plugin tries to find the location of .desktop file the following way: "file = g_strconcat ("applications/", player_name, ".desktop", NULL);"
MediaPlayer2 interface has DesktopEntry property. Maybe you should support this property and use the current code as fallback?
Support for the DesktopEntry property was added with this commit:
Thanks for the bug report!