In bolting fiberglass to stainless plate, under the necessary torque for a friction joint the fiberglass will not be able to maintain the strain required to keep the friction load high enough to design the connection around it. The fiberglass will yield to the pressure. The strength of the joint will be maintained by the shear stress of the bolt. The thickness of the fiberglass (and wood if fastening to a bulkhead or knee) must be enough to also resist the shear stress.

I agree that burying chain plates is a mistake. They should be accessible from inside or outside. My Allied Princess had removable trim to access them from the interior. An important factor also is the seal at the deck for those that are mounted inside. The caulk joint should be thick enough to accommodate flexing without tearing the seal apart. I ground a chamfer in the deck around that chainplate openings at least 1/4" wide and deep. The chainplate could flex with rigging strain and boat movement as much as 1/16" without breaking the seal. Rebed yearly.