Just checked it, there's no possibility to define a X and Y in any of the events files, so that would still need Python.
Some thoughts about the mod component: Obviously you'd need to define a civ, a round to spawn, the type and number of units and the location.
In case no location is given, I'd make them spawn in the capital.
Else I guess peace and open borders have to be respected, as well as spawning within enemy stacks or cities (in that case a test for neighbouring plots would be needed; if all fail, then no units are spawned).
I'd not care about the terrain. If you want units in water or on a peak, then it's your decision.
Did I forget anything?
Promotions, just for the sake of having them
