I think this topic should probably go under inventors corner, but to try and answer your questions:
Most pro doll makers use fiberglass molds to cast silicone dolls. But for a one-off mold you could just make a brush-on mold of silicone or you could use a brush-on plastic mold material like Shell Shock from Smooth-On.http://www.smooth-on.com/Silicone-Rubbe ... index.htmlhttp://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1165/index.html
The advantage of using silicone is that it is flexible and can be removed easily from the model. However you will also need to make a mother mold to keep it rigid during the casting process. Depending on the configuration of the model, the mother mold will probably need to be made in several pieces in order to be able to remove it from the model. One disadvantage of using a silicone mold is that you will need to use a mold release to keep the casting from sticking to the mold. The other is cost. It's about 90 bucks a gallon.
If you used the brush on plastic material, I would think the process would be similar to making the mother mold. Since the mannequin is hard and the mold is hard, you would have to carefully consider the undercuts and design it such that it can be removed from the model. I would then think that you would assemble the sections after removal to form a basic 2 piece mold. You would also need to thoroughly apply a release agent to the mannequin before applying the plastic. The plastic material is about half the cost of the silicone, so that may be a good way to go since you may make many mistakes along the way.
I'm not sure about the materials you mentioned, by rubberskin do you mean silicone?
I would probably go with a platinum silicone skin either Ecoflex 0030 or 0010.http://www.smooth-on.com/Platinum-Cure-Sili/c1115_1130/index.html
You could also use a thin skin of Dragon Skin (painted on the mold before closing) and then fill with the Ecoflex. The Dragon Skin having a higher tear strength.http://www.smooth-on.com/Silicone-Rubber-an/c2_1115_1129/index.html
Smooth-On carries several different types of rigid and flexible foams in several different densities. As far as I know they do not require heat curing. From what I've seen in my research, the foam core is usually made separately and inserted as part of the skeleton system before the silicone is poured into the mold.
Take a look at the videos here:http://www.youtube.com/user/siliconefutur/videos?sort=da&view=u
There are 38 videos showing the process step by step to create a silicone doll from a mannequin. The videos cover everything from preparing the model to molding to casting and even some on making the skeleton. Unfortunately all the titles are in french, but you can always stop the movie and type the text into Google translate to get the basic ideas.
Good luck with your project!