That works!
To elaborate a bit, I go to frame A, Key Dope Sheet, select the frame column, copy, go to frame B, paste. This is much easier than going further back in the timeline to find and copy over keys that are contributing tweens.
The downside is that it would add extra keys to my dopesheet that may not be needed (as described by Pharan). But for my workflow, the tradeoff is worth it.
And then I thought... there should be a way to clean up the unneeded keys at some point later. And in fact, there already is: Animation clean up
It is a nice thing about mature software like Spine that most things you want are already there.
Thanks, Nate.