Here's the situation: the player had a creature dominated. The module gave the list of standard, move, and minor actions the monster had in the standard monster block. But, there was a separate section in the module where it said he had the ability to modify the terrain (he was a giant in control of his tower, so he could open holes in the floor and cause other terrain issues). This ability could be done once per turn as a minor action (but wasn't listed specifically as "at will", just that it could be done once per turn).
Because this was not listed as the giant's powers, the GM said that the player couldn't use the special section as his dominate action. From a rules-as-written perspective, is this correct?
This is a more specific version of this: What can you make a dominated creature do?. And here are the rules for the Dominate effect (from that question):
The creature can't take actions voluntarily. Instead the dominator chooses a single action for the creature to take on the creature's turn: a standard, a move, a minor or a free action. The only powers or game features that the dominator can make the creature use are ones that can be used at will, such as at-will powers....