I think I’ve cracked it - photo below. As suspected, that Wichard shackle block is useful for providing mechanical advantage and is long enough front to back to keep the furler in alignment when under tension. As usual, a low friction eye or two also play a part. I think that chafe may be a bit of a problem, so I’ll use shrink sleeve on some of the more vulnerable bits of string.
If I let go the Wichard line and the continuous furling line, plus the sheets, I can bring the furler into the cockpit quite easily. Then drop the halyard to change the foresail. When done, re-tension the Wichard line, raise the halyard, re-attach the back of the furling line and sheet in and it’s done without too much risk of tangles (or falling overboard). I think when solo this could only be done at anchor or when heaving to but it opens up all sorts of possibilities in fine conditions and when without access to dry land.
Just to clarify, if it all went pear-shaped at the very front for whatever reason, I still have my self-tacking jib holding up the mast. I don't think that it would be feasible to enjoy the convenience of this Wichard arrangement on its own as the mast would probably end up in the cockpit, at speed.
