OK, I have an enhancement to the ExternalEvents feature. If you configure it with droppable : true (It's as simple as that if it is using a Grid because we understand a Grid) then you get a dropExternal event fired by the Calendar.
API still to be discussed internally, but the concept is working:
It's a bit too much to patch. This should be available in 5.4
droppable : true will be on the feature