Fiberglass resin may melt the thin plastic wrap. I would experiment with it first.
As for body casting, unless you have the money or need for skin detail I wouldn't do alginate. Straight plaster bandage will be just fine.
I personally did mine with ultracal and burlap(I know people say not to) I didn't have any heat build up,no burning or anything bad. Like Ink said, it will be thin enough to where it doesn't generate the heat like it does with a mold.
I used plastic wrap and wrapped my whole body first, you can also get a leotard and coat it with oil or vaseline.