It did work... that is, until it didn't any more. Pinkysil IS very forgiving but it can only handle being stretched to billy-oh for so long. Even though I cut down through both sides of the mould to gain access, the interior voids of the nut were quite intricate, and extricating the plaster cast required some degree of force. After only a couple of casts, pop!
Still, I did manage to get three decent casts from it before it busted. Never mind.. this is one of the perils of mould making. Sometimes you make it one way only to realise it would have worked better another way. For example, this one would probably work better as a two-part mould, but even so, interior spaces are difficult to deal with, and it still might not have worked.