Well, I'm certainly no fiberglass expert. I do feel very comfortable with the backing plates I just glassed to the hull. I did a moderate amount of research, and decided to use Evercoat Kitty Hair with the Groco backing plates. A few notes:
1) The Mainesail article
Seacock Backing Plates - Marine How To says "Whether epoxy, polyester or vinylester all should be plenty strong for this application." although he recommends using backing plates made from "GPO-3 polyester/fiberglass or G-10 epoxy/fiberglass".
2) In his other article
Replacing Thru-Hulls and Seacocks - Marine How To, he uses Kitty Hair, the same product I used. So maybe this is a horrible mistake (using a polyester resin fiberglass where it may get wet), but at least I took expert advice in using it.
3) In the boats.com article about basic boat building practices,
Boat Building: Basic Construction of Resin, Fiberglass, and Cores - boats.com, they say
"There are three types of resins: polyester, vinylester and epoxy. ...Polyester: This is the resin most commonly used for boatbuilding today, and most boat owners are familiar with it. It is inexpensive and generally all-purpose. It has low stretch (elongation) properties so it is not used on modern high-performance boats, but it is perfectly adequate for most boats. The most common polyester is an orthophthalic base, but newer isophthalic based polyesters are gaining in popularity. The isophthalics are more resistant to water and chemicals, are more abrasion resistant, and have higher impact and fatigue (flex) performance. Most modern gel coat finishes are made with isophthalic resins."
4) I did worry about whether this particular product (Kitty Hair) would adhere to the Groco plates, but some research convinced me it would probably be fine. These plates bear no resemblance in texture to a smooth PVC pipe: they have a very rough surface and they're being bedded with a filet around the edge. They are also being firmly secured between the thru-hull and the flanged seacock. It's not like I'm attaching a pad eye to the backing block and trying to pull it off: it will be under constant compression in this application. So far (prior to securing the fittings) they seem extremely well adhered. I assume that using gelcoat to finish them (which adheres better to polyester resin than it does to epoxy resin) will help with the waterproofing.
5) As for bolting the flanged seacock to the backing plate, this is recommended as an option by both Groco and Mainesail (in the first article on backing plates: "Many readers have asked me how to install stronger,
flanged seacocks without also drilling extra holes in your hull for the bolts that hold the flange. It can be done, as you will see below." I certainly understand that this isn't as strong as bolting through the hull: the main point of it here is to keep the flanged seacock from rotating. And the main function of the backing plate is to distribute load. It's a product made by one of the best fitting manufacturers out there, and I feel comfortable using it.