Nice ideas! This is fun to think about, even without DF in mind.
I like Blastbeard's immune system idea, but with different justification. Good isn't a point of view according to traditional fantasy RPG rules. I was never into pen and paper, but I know from computer DnD games that Paladins and Solars and the like (lawful IIRC) are pretty much required by their alignment to be mortally opposed to anything evil-aligned. DF clearly labels "good" and "evil" regions and has an explicitly medieval setting. So I take the moral scale to be pretty absolute (with dwarves, elves, and human civs on the whole being somewhere in the neutral zone). Maximally good beings are more than just nice and friendly. They know exactly what evil is and will try to wipe it out. It's not just violence stopping violence, it would be more like righteously driving vandals from a temple sanctuary. You've arrived in their immaculate paradise with the stench of moral compromise about you, pulling wagons of slaughtered trees, the bones of mutilated sheep dangling around your necks. The region's residents will consider violently purging even slightly evil beings to be a good act, fully justified (even required by duty) since it would restore the region's purity.*
Yeah, I don't want RL humans acting like this because we're so far from agreed on whether good and evil even exist, let alone what they are. But in the hypothetical context of there being a real, knowable Good, I'd fully expect its legit representatives to act this way.
I'm not sure I'm okay with good regions altering the minds of those who stick around long enough. For whatever reason, I consider free will to be integral to their being a valid difference between good and evil. Without a choice, acting one way or the other has no moral value. So if a region "compels" you to do good (in gameplay terms) I'd want to see it reflected in the dwarves listed personalities and preferences, and in a way that suggests not that they're becoming mindless cuddle zombies, but are inspired by their new environment to strive to be more humble, calm, and empathetic. This would bring in a justification for darknessofthenight's idea. Your veteran dwarves might eventually be so transformed that instead of taking joy in slaughter, they will instead be horrified at themselves for killing something, even by accident or in self-defence.
Maybe only dwarves born in the region are capable of becoming fully good. Maybe they're born fully good to start. Maybe along the DnD alignment end, catoblepas' idea for region personalities could be minimally fulfilled if the current chaos axis wasn't just "calm to wild" but was more like "chaotic to lawful" with calm in the middle. Lawful would be the "purge all evil" type. Chaotic might be more of a hippie love-in pacifist thing which would be much easier to take advantage of economically, but wouldn't offer the protections of a lawful region, and the locals might not share dwarven concepts of private property and personal space (some of that in good regions already).
* I realize that perhaps this kind of agency should only be found in the "higher" creatures (humanoids and the like) while "lesser" creatures might act instinctively or indifferently. It's a complex game, right?