hellosailor - Thanks for the question. The series/jordan drogue is trailed from the stern. You use a bridle (well a split bridle) from as aft and as wide as possible. That is why the horizontal chainplates at the transom are handy as they are wide and aft.
goes from the port chainplate to a free rotating shackle - and the same from the starboard.
The reason for the exterior plates is also to avoid chaffe. It is recommended that the chaffe is so great that the bridle will risk being lost
So if you splice and eye (around a metal eyelet) at the end of the bride the chaffe wont occur.
One onther thing I have noticed. Last night I decided to go out and look at my transome (what a difference look at it makes). My hull comes up past the deck
to form a small wall around the boat. This is about 30cm high and 7 or 8 cm thick. This is much thicker that the hull (3ish cm). A bolt through the 'wall' would disperse its load of much more cement.
Also consider the catastrophic failure mode - the chainplate is ripped out. If this occurred in the 'wall' then the damage to the deck-hull cocoon would be less (or nothing) compared to it ripping out of the hull proper.
However you suggestion did make me think of using a chain strop from ondeck, through a fair-lead and out to the rope
bridle. I could then use my post cleats
(they could just have extra backing plates and easily be over engineered for the job). Some quick calcs show that my panama
fairlead has about 60% more contact area with concrete that 12 x 20mm bolts - so good for the concrete. They are also in the 'wall' so I have the benefit of the extreme failure mode being outside of the waterproof cocoon.
One other note on failure modes. I want the fixing to the boat and the passage
of the load out of the boat to have a wide safety
margin so that they can easily take any load I will throw at them. However I will design in a 'safe' failure mode by joining the strop to the boat via a shackle with a breaking strain much lower than the boat fixings (but still above the max load) so that in some kind of extreme or unpredictable failure the shackle will go before anything else.