Thank you to everyone who helped with suggestions and comments.
Orchid proves its worth again!
Looks like the most effective methods I’ve come up with, or been
offered suggestions for, all involve the tapered shaft driven into
some sort of articulated shell inside the ring, i.e., the exact same
principle as the ring stretchers I have on hand. It is possible that a
new one could be found that starts out at size three, but even that is
too large.
One suggestion was to lay strips along the shaft of the one that’s
too large, extending beyond its tip and coming to a smaller point that
will slip inside the ring, then driving the strips back up the taper
until the ring fits over the end. This seems like a very
straightforward and effective strategy. Thanks to Drew Horn for this
idea.
A related device would be a bundle of very small rods, enough to
almost fill the inner diameter of the ring, with a small tapered shaft
inserted and driven in to expand the group’s diameter incrementally.
The taper could be withdrawn and more small rods added, as necessary,
until the desired size is reached. The ring could end up out of round,
by this method, but slipping it onto a billiards cue and rolling it
out would fix that. (One of my favorite tools – five bucks at the
local department store or sports store for a cheap solid cue, cut it
up and make all sorts of stuff out of it. 
There is one other solution, of course, and that is to file down the
leaves on a regular ring stretcher to reduce its tip to a size that
will go into the ring. This is a little drastic, as it permanently
alters the tool, and it’s limited in that you can only remove as much
material as there is present in the first place.
Loren http://www.golden-knots.com/ lorenzo@intnet.net
@Loren_S_Damewood1