My vote -- since it's what I did -- is to fill the seam from the outside. I was addressing the problem of leaks rather than trying to reinforce the joint, and may have accomplished both. The leaks are gone.
My Ariel has a teak caprail bolted through the toerail. I simply removed, bent up and threw away the stainless rubrail; scraped and ground out the silicone goo and dry cloth in the joint; and pressed epoxy thickened with West Systems 406 into the joint and faired it out. Will paint the hull next summer.
Since I can't see the joint from the inside, it may be that somebody already reinforced it - but it all looks original to me.
For what it's worth.
--Dave Gillespie