I have come across some interesting discussion and reviews of a product called Instant Mold, a silicone like moulding product that softens when heated and does not stick to modelling putty, allowing items to be cast in Green Stuff and similar.
This video shows it better than I can describe but I would caution against his modelling knife using skills:
Roebeast has a more in depth assessment of the product on his blog here which shows some of the uses he has put it to which certainly shows applications of the product off better than the video.
Whilst Instant Mold is available from CMON there is an equivalent product available at half the price (I have read that it is the same product but can't confirm); Oyumaru Instant Mold Moulding Compound, available in the UK from Metal Clay Ltd for £4.95. At that price it has got to be worth giving it a shot as the potential uses seems limitless...
Thanks to Andrew Wood and Ian Rees for the heads up on this.