Just did a pumpkin ale exactly like you are describing. I baked the pumpkin at 350 on a cookie sheet, collected the pumpkin matter and added it slowly at the start of the boil. With a bit of stirring the pumpkin pretty much broke up and was not heavy enough to sink to the bottom. I had no scorching at all on the bottom of my kettle.
Watch the baked pumpkin though. I spread it very thin and it make it difficult to get some of the pumpkin out of the pan. Either keep the layer a bit thicker (around 1/4 inch) or bake at a slightly lower temp and keep your eye on it.
Primary:IPA, Belgian Dark Strong Ale
Enjoying: Gumball Head, Belgian Wit
In the Works: