I would not worry about it. What is your fear? You are cooking the pumpkin to break it down, get it out of the skin and make it more accesible to the enzymes in the mash. Even if the pumpkin introduced bugs/bacteria to the mash, it would have to sit for ~12-24 hrs to become a sour mash.
Cook the pumpkin, scoop it out, cover and put it in the fridge. The biggest issue is managing your strike water with the pumpkin in there. I used my next day, but put it in an oven set to 160° for the hour before I mashed in. It was perfect.