Just to finish this out, my Grobet USA 6" cut 4 file arrived, so for my second attempt, I used the file on the Wilson Combat Bullet Proof slide stop I'd bought. It took about 30 minutes of steady filing, stopping to check the fit from time to time. After I got it close, I rounded the pin free hand, just gave it enough of a bevel to allow it to insert easily into the frame.
I blued the pin ends of both parts with some Birchwood Casey Presto Mag. I inserted the WC BP slide stop because it seems a bit more robust than the factory SA part, and it has the pre-filed logman / idiot scratch prevention divot where the plunger tube sits, making assembly easier.
I am pretty happy with the end result; the blued color, even with just the one application, seems to match the frame tolerably well. (*)
(* This was my first exposure to using Gun Blue. It was pretty interesting to learn how this stuff works. I felt like an Alchemist. )