This is the kind of thing that boat-builders are doing all the time, but I can't think of case where I've ever seen a plate embedded in laminate. For one thing, polyester resin isn't a very good adhesive so it won't bond to the plate - I suppose that you could put holes in so that the laminate is bonded through the plate, or use an epoxy laminate? Fixings to take rigging loads on a GRP boat would generally be through-bolted and use a sealant like Sikaflex which would give a good working life and if properly made, guaranteed waterproof.