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: https://git.xfce.org/panel-plugins/xfce4-pulseaudio-plugin/commit/?id=c02ece5b1fc415a5379fdd17133249fedc09f1c6 Thanks for the bug report!