Chris,
I'll strongly ditto Lee G on this one. Just slap another swatch of #6 F/G on the underside and be done with it. Remember just because its glass doesn't mean its fragile Yeah you could use all kinds of industrial strength addditives, but you also could build a steel kayak too. At some point, your building something that is more survivable then your own body is... which is also part of the boat equation. The additives are likely going to impact the look of the wood your glassing over Which is a trade off that I don't think is necessary in this case.

Chris, I have a similar like for a way strong hull, and a need for abrasion resistance. I am a rock garden afficiando. In fact I am cleaning up today from a trip to the Cape Nedick/Marginal Way area that has acres of some real cool boomer fields. I punked and poked intentionallly see-sawed my boat on a few rocks after a wave passed waited for the next one etc.Surfed in a estuary entrace and pullout above a half mile of muscle beds.

For me the answer was as simple as upping to #6 inside the hull and two layers of #6 on the outside. I'm getting the strength and abrasion resistace I need. And, despite the rocks and barnicals doing their very best, I've got good coverage underneath.

Yeah at some point I'll probobly have to sand off the outer layer of glass underneath and put on a new coat of # 6 but it s a long ways off and its not going to be a very taxing process. For me it its a Kiss solution that doesn't detract from the look.

See ya in the Rock Gardens