[itsme]

How about storing the formatted key as string in the database? Only for ChordPro songs with a transpose value specified. It could be clearly specified when these entries have to be updated: for all ChordPro songs when the change in the database appears for the first time and when 'Chord Localisation' is changed. Maybe there are more conditions that I'm not aware of. And the respective formatted chord string has to be updated for individual songs, which is not time critical at all. This is the case, when the transpose value of a song is changed and when a new ChordPro song is imported. That seems pretty feasible for me.
As soon as these formatted keys are available in the database they could be displayed like any other database field.