Why rotocast this piece? You'd save so much time and material making a two piece mold.. in simple terms..take the "bowl".. clay off the part to the desired thickness, and make a plug.
You up seem to be hung up on Free Form Air as well. A simple mold like this really wouldn't need it.. stick to fiberglass. It is strong enough on its own to make a working mother mold.
Side note, the amount of clay has nothing to do with whether or not silicone will cure. It has everything to do with what's in the clay. As long as you have a sulpher free clay (Monster Clay, or even better White Clay) you can block off the top and mold without issue.
I'd be happy to walk you through the process or answer any questions you have.. anything to help.
A great idea, in theory. However, and this may sound counter intuitive, but believe it or not an indent like that will use more resin than a simple dome shape.. surface area, and all that. That also goes for materials in molding as well.
I wouldn't recommend using a real helmet as a plug, simply because it is metal. In theory there are no undercuts to lock them together, and you can "grease it up," but having hard metal contact hard resin scares me. You could make a proper mold and plug with three trial kits of Rebound and some fiberglass. Costs could be as low as a few hundred bucks.. definitely not $500.