I considered doing something similar, but I didn't like the idea for two reasons. First, it struck me as a pretty expensive use of plastic on the printer - to do the torso I could easily see burning through 3-4 reels just to create a mold. Secondly, even if you're printing in ABS, isn't the fact that you're using a heatgun to form the worbla likely to end up warping the mold that you've printed, too?
On the other hand, if you come up with a model to use as a form for the worbla, I'd love to see it shared because as I think about it, it'd be an interesting exercise to try anyway.