They have always been immune to stun and flashbang even before the experimental patch. Bear trap is the way to go but if you don't have quick tinkering than try caltrops/toxic gas grenades.
Ah. The much lower enemy health on Hard (and the substantial rarity of finding Ancient Rathounds) meant I didn't have any strong memories of trouble with them. I must have forgotten that they weren't stunnable.
I do have Quick Tinkering, indeed. I'm finding that I have to have a whole lot more combat-relevant feats on Dominating than on Hard. A crafter build is a little lacking given that it only looks like I'm going to be able to afford two crafting feats. Still, Dominating is an enjoyably difficult experience with only very few "Aw, bull@#$%" moments.