I'm trying to think of ways to take advantage of the fact Overlord usually goes second, and I'm also strictly talking about Setup, not reinforcements. Cardinal's Plight is a good example.
In the Quest Guide, on pg. 2 is states in the 3rd paragraph under Setup:
Monster figures may be placed anywhere on the tile or tiles indicated in the setup instructions, provided that those spaces are empty. If there is not enough room on the map tile for all the monsters in the group to be placed, excess monsters may be placed in the closest available empty spaces.
It doesn't mention anywhere that the OL cannot cannot spill into other tiles. If this is against the spirit of the game to some, I understand, but I'm against players that are very cunning/wiley and they think of the craziest things sometimes.