Solent Stay

[Brion Toss]

Hi,
Several riggers? I ask, because, while it seems possible that there is one wingnut out there who might suggest such a thing, I'm surprised that there might be several. I can't even recall ever seeing a backing plate installed for this application; ordinarily that is reserved for boom vang fittings, where the loads are relatively high, on a much thinner section.
There are internal tangs, sometimes used for forestays, that fit inside the mast. These are not backing plates, but the whole thing, and there are limits to their articulation, and thus to their use for a stay.
If you have a well-shaped fitting, adequately fastened, there is ample support to oppose the pull of the stay. I am working on a boat right now that has been around the world -- twice -- that doesn't even have a throughbolt for this fitting.
As for doing this in place, while it is possible, I urge you to take it out, so you can do the job much faster, easier, and safer, with the stick on sawhorses.
Fair leads,
Brion Toss