This turned out to be a TERRIBLE mistake.
PSA: CUSTOM PANELS ARE UTTERLY BROKEN IN 19.10. DO NOT TRY TO LOAD ONE.
I saved the panel layout (not quite the same as the one above, but close enough for this), switched to one of the built-in layouts (SUSE, chosen at random, since all I wanted was "not what I have"), and switched back to mine. What I actually got back was two panels that were mostly empty, with EVERY setting for EVERY applet wiped out.
The MATE Menu and Control Center were missing entirely, but that was the least of my concerns.
System Monitor was back to CPU-only and default colors (which I'll come back to). Only 1 CPU was being monitored. The correct names for the HW sensors had been replaced with "temp1" etc. My location (for Weather) was gone. And so on. You get the picture.
I tried killing and restarting mate-panel, and even logging out, just in case those would get it to recover, but no such luck.
It took about 45 minutes to get everything reconfigured. Not happy. I know it's still Beta, and I signed up for problems when I installed it, but ugh. Then I saved it under a new name and tried to load that one, and that barfed in the same way so I gave up.
Oddly enough, the panel from 16.04 has been loading fine, aside from the bug that started this thread. I should have quit while I was only slightly behind...
I've got the config files, and I'll dig into them over the weekend.
On a related note: mate-panel needs to save the config files under the user's home directory, not in /usr. How is a normal user expected to copy them between machines, or restore them from a backup?
System Monitor I actually COULDN'T get reconfigured properly, because, wow, what the @#$% is that color chooser supposed to be?! It's not just dumbed down to near-uselessness compared to the old one, but it's broken AF on top of that.
Can we please have the MATE one back, with the wheel and the eyedropper? I don't care if it only works in the MATE apps. "But it wouldn't be consistent then" is irrelevant when Option B is "utter garbage".