I also experienced that re-boarding can be pretty awkward.
I made a somewhat similar, but simpler and not-always-deployed approach to that of TonyStott. I have a length of line with bowline loops at both ends. One of the loops has a 5 inch length of irrigation hose on the rope of the loop to hold it open as a stirrup. The other loop I pass over the aka, and pass the stirrup end through, to secure it to the aka, hanging down. Then, reboarding involves getting this out of the mesh pocket where I stow it, deploying it (can be done in advance if the swim is planned), stepping in the stirrup, and up to the boat or tramp. It can slide along the aka if there are not tramp-buckles preventing that, and I think that could be minimized by a longer upper loop doubled around the aka, but that may be unneeded.
Mine has the disadvantage of not always be in place for quick emergency use, and the advantage of simplicity and neater stowage when not in use. It is made of lighter line, less comfortable but more compact, and plenty strong for the purpose, especially since mine has the pull directly along the rope.
I did not understand Tony's statement that because the outer loop was a bowline, one could adjust the length of the line while in the water. Resizing a bowline seems to me to involve untying it and retying, doable but not special to bowlines, and not something I'd envision doing, especially in urgent situations. Did I miss something, Tony?
If you found yourself without such an arrangement, you could probably use the mainsheet, with the sail reefed and cleated, and the length of arc hanging down into the water adjusted by cleating the mainsheet in the right place. I have not tried this, but now that I have thought of it, intend to try it as an emergency drill. For a big guy like me, it will involve quite a pull on the sheet, on both reefing and main cleats and on the sail clew attachment, more than these experience from the wind, I think, so I don't think I will make a habit of it. I don't weigh as much in the water, though, especially salt water
. This would also have other problems: if you had tramps, would you be able make a big enough loop behind or in front of the akas? It will necessarily be right next to the hull, and have the problem Tony mentions of swinging under the boat.