I just finished fermenting a Belgian dubbel with around 4 lbs of pumpkin added to the mash a few weeks ago (can't remember which Belgian strain I used off the top of my head). I'm going to do some bench tests on a pumpkin pie spice tincture to figure out my dosing for the full batch. So, I don't see anything wrong with your idea as I'm doing the same thing. My thought process is the same as yours, I bet the Belgian yeast character would go well with pumpkin spice. But, I'm in the middle of dry-walling my garage so the beer is sitting in the primary clearing...hopefully I will get around to playing with it in the next week or so as I want it ready for Thanksgiving.