It needs to go in a themed sale, though, doesn't it? I wouldn't suggest a job lot, but if it doesn't have any authentication (i.e., it could be any old bit of plate inscribed by a fan or a fraud!), it's best in a themed sale or offered to Sotheby's, as was suggested. Its problem is that it has no provenance to speak of, i.e., it's not being offered by the horse's connections.
Muttley, thinking about authenticating such items, how would one prove that a person had NOT just bought an appropriate piece of plain plate and had it engraved falsely with what would be 'worthwhile' information on it? For example, what would prevent me from buying a big crystal bowl at an auction, having an engraver put 'Sea Bird II - Winner, The Derby, Epsom, 1965, ridden by T.P. Glennon' on it, and offer it for auction with some barmy reserve on it? And if I trawled a few junk shops, I'd possibly find a well-aged large box that I could bung it in, to make it seem more authentic. Are such faked items often offered?