For Plasticine you state that: "Although...quite firm at room temperature, it is still not rigid enough to use as a mould material in most situations. Any gelcoat applied to it would be liable to cracking if the mould flexed when reinforcement was added"
The situation would be quite similar for wax so, much like plasticine, we wouldn't really suggest using either of these materials to 'sculpt' a pattern which you will then lay mould making materials (gelcoat and reinforcement); however, it would depend on the size and shape of what you're making and how careful you were with the layup process. I could imagine being careful enough with the gelcoating and laminating to use either of these materials as a sculpted pattern if one was very careful about it.