It looks like the epoxy plug idea is going to work. Made a mock-up of components to the correct dimensions and tried a couple of options: a) 6mm enlarged hole b)9.5mm enlarged hole.
Both were drilled 7/8" deep in the 1" deck simulation and filled with epoxy using a large syringe - so I could fill from the bottom up and avoid capturing air bubbles. Left to cure overnight they were drilled out to 5/8" with a taper drill for the screw pilot hole.
Meanwhile a piece of the toerail was countersunk to a depth of just under 1/8" and the rail plus bobbin screwed into both test sockets. The test piece was dis-assembled and then sectioned with the bandsaw to see the results:
The bottom is the 6mm socket. Here the screw just about breaches the socket sides, though the depth seems fine. This is not what I want, because there is a slight chance of water entering the wood fibres. In addition, the accuracy required to centralise the pilot hole exactly is going to be time consuming...
The 9.5mm socket accommodates the screw comfortably with plenty of lateral thickness spare.
The only slight issue is that there was quite a bit of absorption of the epoxy into the wood (good), this however leaves a surface depression (bad). This either means I have to make the epoxy thicker and overfill more than I did, or top up later...
|Filled socket with pilot hole drilled awaiting bobbin & screw.|