So, getting back to try a different approach.
Rather than operate through DFHackery, I decided to try using advanced worldgen parameters to spin up a small worldgen paramset with minimal features - a flat plain, with glaciers in the north, tropics in the south, and flat plains in between. I removed all megabeasts, night creatures, secrets and savagery in an effort to make the setup as invariable as possible. I then created two civilizations, Ice Giants and Fire Giants, both of them physically identical, with no need for food and drink, and gave them the ability to settle anywhere; differentiated only by one starting in the polar regions and the other in the tropics. The perfect setting to watch their civs spread across the world and clash in the middle during worldgen.
From there, I began the process of adjusting their traits in an effort to see what would impact worldgen events.
Adjusting ethics to make them opposed to each other worked fine. If their ethics were similar, wars rarely occurred in worldgen; if their ethics were very different, they would be more likely to go to war.
Differing VALUES didn't appear to do anything noticeable.
After a very many, many tests, I finally found something besides ethics that seemed to make an impact. IF there was hatred due to differing ethics, giving BOTH sides max-level CONFIDENCE, BRAVERY, VENGEFUL, and AMBITION seemed to increase the chances of conflicts when the two civs met. This could be viewed by the expansion of both civs grinding to a halt as they met in the middle, the world population would show a noticeable increase in deaths, and this would often cause forts to pop up along the front lines. Sometimes, one civ would end up taking over the other. If these personality traits were low, it would be more likely that the civs would simply cross over and build sites on each others' territories, even if they hated each other.
Notably, if only ONE civ had these traits, they would not clash visibly.
This is, so far, the first evidence I have found of personality traits making an impact on worldgen behavior.