The job is done, and turned out wonderfully.  Many thanks for all your suggestions.  You can read (and see) it here https://tammynorie.wordpress.com/2014/06/11/making-the-mast-hinge-blood-sweat-and-duralac/

Thank you very much for your suggestions.  I think they could help, but they don't deal the with the main difficulty: the mast wobbling while you're trying to drop the sleeve (or just after you raise it).

Picture me standing at the mast stub holding supporting the mast with my body (and the boat rolling on the water, too).  If I push in spring pins or screw in extension bolts and there's a bit of force on the mast then pop! out comes the pin and the mast goes overboard.

And Mark, that's also why it matters and not anything will do.  The hinge needs to be strong enough to support mast's own weight, forces during raising and lowering, and forces from a bit of rolling.  At the moment it is not.

We had a thread about this on the Cambridge University Yacht Club mailing list and my friend Igor came up with a nice suggestion: a couple of these flanges inside the mast stub, rivetted or bolted, effectively grasping the pin inside as well as outside the upper mast section.  They should stop the pin escaping, and rolling forces on the mast are transferred along the length of these flanges.

This is pretty easy to make, and doesn't require any complicated mechanisms or handling.

What do you think?

I've drawn a little sketch of an idea I might try, involving spring-loaded pins which can protrude much further from the mast when the sleeve is slid off. Need to be well lubricated though. I'd probably fabricate it in brass or bronze.

Might tend to bind too much with the weight of the mast. Another idea might be to thread the two pins into the body with left and right hand threads, so that rotating the body would draw the pins in and out.

In both cases the 'body' of the pin would prevent it dropping out as long as both ends protruded a little. The 'ends' could be slotted so a screwdriver could be used to adjust their depth. In this case, both threads could be the same, like this.

If you cut the unthreaded portion of the bolts too long and screw them both fully home, you can the mark the final cut flush so the sliding piece fits over. That way the pin body can never drop out unless you remove the screw pieces completely.

Chris