First of all, the loads are low enough on a Pearson 26, that unless you are talking about backing plates for the keel bolts, backing plates are not really necessary if you use fender
washers. But beyond that, I would say that if you actually have fiberglass backing plates, in my opinion that is as good as it gets.
Stainless steel makes less than no sense at all as backing plates. Of the other recommendations, IPE is a good choice because it rot resistant behaving a lot like teak (no need to epoxy
saturate it). Plywood work okay if it is a hardwood and epoxy
saturated. Oak is a traditional material, but I have removed lots of rotten oak backing plates over the years. 1/4" or 5/16" aluminum actually works quite well and is easy to machine in place and can often be purchased cheaply as scrap from spar shops.