Change policies fro a specific civilization

Basically, I'm working on a civilization who's UA will grant it a research boost upon finishing a policy tree, and I'm unsure how to implement that. Nothing in the trait tables allows anything like that, and the only way I seem to be able to manage it change the policies directly, but there's also no way to change that for only a certain civilization.

I guess my problem is a lack of anything to reference. Does anyone have an idea of how this could be done, or perhaps a mod that does something similar?

That should be pretty easy. There is an event that fires when a player adopts a policy. I don't think it fires for the finisher policy so what you'll have to test if the adopted policy was the last one in that tree. If so, give the player some science.