AFAIK, sealing exposed core is unusual for boat builders. Most just drill/cut and ignore it; the premium builders "pot" all holes with epoxy and countersink the holes. Pearsons have above average construction, but they were not a premium builder!
I have come across some builders using solid epoxy blocks in areas where the hardware is mounted, but that is rare. Pearsons sometimes had marine ply in areas where extra strength is required.
Personally I used a dremel with a router bit to cut out ~1/8" of core, while leaving the top and bottom fiberglass "skin" intact, and fill that space - firstly with plain epoxy to penetrate the wood; then with a thickened epoxy to fill the space.
You will indeed find many applications for epoxy...but the good news is that it is easy to handle. Remember to add fillers where appropriate - nothing (for penetration), carbosil as the thickener - prevents sags/drips - various grades of chopped fiber for strength.