John, the base plate that you can see in the picture is 1/8" aluminum. Bolting a padeye style fairlead right in front of the mast and running a spectra loop is possible. Other than re-engineering the hinge to get rid of the sleeve (or basket as you cal it) to free up the front of the mast, or drilling for a cross bolt, all solutions discussed so far end up with putting an upward load on the deck. I'm confident that I can solve any issues of local stress, the base-plate arrangement with its plywood backing plate is sufficient.
Leaves the question whether a deck that can handle 2-300 lbs crew weight during rigging will stand the same force when applied up not down. K.C.'s estimate is reassuring in that he's not maxing out that 20:1
-- in fact what he reports is below the reverse of crew weight, leaving a reserve for peak loads (sudden gusts).
The through bolt approach has the advantage of simplicity and completeness (just the bolt and the bail), but the downside is that I'd have to assemble it each time - my efforts in general go towards reducing the effort for setting up.
During the season, for example, my jib-sheet stays rigged as shown
the light green is a soft shackle tied with a Prusik knot to the sheet. Even though the shackle is slippery Amsteel and the sheet also has a rather slippery cover, the knot has never moved since I tied it. With this setup, I simply connect the clew.
A similar shackle,or even a spliced loop could be used with a padeye, and be left in place to attach the vang. Amsteel is super-easy to splice... if experience shows too much "give" in the deck, there was the suggestion to rig what amounts to a reverse compression post. Simply a strong strap connecting deck and keelson. (Except, I have a mast-jack on my DS1, so I could not run anything close to the mast base, or I can't turn the bronze nut that raises the mast to tension the rig.)
Several people have reported here that they built a sturdy frame connecting the rear lip of the cuddy to the floor, to stiffen the boat. So, that is something I could consider making part of the overall design. (As much as I hate encumbering the cuddy opening).
It occurred to me that the bolt & bail setup might be improved if I could fit a bit of aluminum tube connecting the holes on the side of the mast, so as to provide a guide for a bolt. That would take the element of "fishing for the opposite hole" out of the equation...
So, back to the drawing board.
Anyway, thanks to everyone who generously contributed suggestions.