I see nobody ever answered your question... I have the same question. Does pumpkin affect the mash pH? I know I can take a pH reading while I'm mashing but I would rather be able to guesstimate a bit beforehand... Bueller?... Bueller?... Bueller?
Unless the added material is at the same pH as the mash without it it will effect the pH. The direction depends on whether the pumkin is at higher or lower pH than the mash (it will pull it in that direction) and the magnitude of the shift will depend on the pH of the mash and pumpkin and the buffering capacities of each. Clearly a test is the simplest way to answer this question.
Just add the pumpkin near the end of the mash and it won't make any difference. There's no need to mash pumpkin anyway since there's so little starch in it. Check out the can's label if you don't believe me.