The deed is done! Had to do more hacks than I expected and may still have a hack or two in my future with this derailleur, but it is silver now:
It’s beautiful! Or at least it’s as pretty as I could get it with the pitting some of the NaOH caused and from me not having anything finer than a 600 grit to sand it down with. Now for all the hacks.
This time I protected the main mounting threads with some putty, as the last solution took an M7 down to an M6 on the outer cage plate and it wouldn’t mount. Somehow the mounting bolt came free from the plate, though (it’s a press fit) and I had to do some finagling to get it to screw in tight to the body of the RD.
The threads I forgot to protect were the little ones that hold the screw that keeps the main spring in tension – the little round buy with the phillips head on all these shimano RDs. The NaOH ate what I guess is an M4 hole up to about an M5. I had to tap it and run a low-profile water bottle cage bolt in there I had lying around from some wolftooth kit. It juuuuust clears the guide pulley:
The other threads I also forgot to protect the first time around were where the tension pulley mounting bolt threads into the inner cage plate. What was (I think) an M5 hole is now about an M6, and until I find a longer bolt that will fit in there, a couple threads (and some threadlocker) are holding a nut on the other side of this bolt.
I vaguely recall buying a ton of nearly-right screws and nuts from various hardware stores on a past adventure getting a front rack to mount on an '80s lowrider fork with un-threaded mounting holes, and while that was a very annoying set of trial-and-error parts store runs, I’m now vaguely happy that I have a tiny drawerset full of various make-it-work hardware.
I’ll see how it rides, and if a longer bolt keeps everything in place or if I’ll have to buy another inner cage plate and just oven-cleaner it.