If they were unpainted I would offer them up to someone to cast up and preserve for BFG community.. I guess I could paint strip them of their black undercoat.
The paint won't be any hindrance for moulding, however like you say being metal they can easily be stripped.
Honestly though other than nostalgia I don't see any point in recasting them. IMO they're inferior to any of the production BFG parts and look more akin to Space Fleet. I suppose the prow is interesting for something different.
But I don't know how GW's IP works if they were un-released models?
Technically they fall under the same laws as any other sculpts, production or not. But I'm sure nobody would complain if a few copies were shared by friends.