The pumpkin itself doesn't contribute that much to what most people think of as pumpkin flavor. Most people think of pumpkin pie when they say "pumpkin flavor," and the pumpkin pie taste comes from the pumpkin pie spices, not the pumpkin itself. No matter how much actual pumpkin you put in the brew, and where you put it, the flavor that comes from the actual pumpkin is miniscule. What you do get from the starchy pumpkin is a unique mouthfeel and consistency in the final brew. In fact, you can make a "pumpkin ale" with yams, butternut squash, or sweet potato along with pumpkin pie spice, because they have a similar starchy protein profile. There are lots of commerical examples of "Pumpkin Ales" that JUST have the spices, and no pumpkin at all (Shipyard Pumpkin comes to mind).
Anyway, all that to say, there is really no reason to add pumpkin in the secondary. At fermentation temps, you are not going to extract much of the starch/protein profile that the pumpkin itself actually adds to the pumpkin ale. Some people add it to the boil, but I personally think you get much more out of it in the mash/steeping grains, plus it is just easier to keep more of the stringy pumpkin meat out of the wort if you use it in the mesh bag, resulting in less trub.