I can image a travel charger where the power supply is outside the charging unit - there are many wall (120/240V) to 12V power supplies. The output side is a round/cylindrical plug. Could be a common with a AA charger, ...
Attached to batter would be a much smaller unit - connection to the LP-E4(N) battery and a bit logic necessary. The latter unit I suspect would be no larger than 1 x 1 x 2 inches. If I knew enough electronics I would find an old charger and take it apart to see if this would be possible.