I wouldn't dehydrate it. I'd cut it into cubes, put it in a ziplock freezer bag, and put it in the freezer, then the fridge a few days before you want to use it.
I think by dehydrating it, you'd actually lose some of the properties of the pumpkin that make it useful in brewing.
Remember that pumpkin doesn't really add a "flavor" to pumpkin ales, it adds a sort of protein base to the body that balances the pumpkin pie spices you will add in the boil.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!