I did this:
property var cachedPlayer : QtObject{
                    function playbackEpisode(id){
                        console.warn("callback worked!!")
                    }
                }
in EpisodeItem and replaced the call with cached player, but I still get file:///usr/share/harbour-org.gpodder.sailfish/qml/EpisodeItem.qml:92: TypeError: Cannot call method 'playbackEpisode' of null

I think the callback is really executed in the context of the dialog and not where it is defined...