Not knowing all the ins and outs of Simplex derailleurs, I linked a photo from Velobase indicating the basic features of derailleurs I was looking at on Ebay, which turned out to be SX410, or similar. All metal, at least.
Main point of my question was answered in the negative, i.e. would it be a bolt-on for the Campy NR. Thanks!

Thanks for the tip! I might consider that if the NR doesn't work out (see below).

Thanks. Both look like good options functionally and for keeping the European vibe. Esthetically though, they seem a little modern compared to the remainder of my build...
I've been travelling and visiting family the past few days, so sorry for my delayed feedback, but I really appreciate the input and discussion.

Now I've had some time to do some research and calculation, it appears that the NR can do the job as is. My gearing combo will be 13-26 and 36-48-52, most likely. The NR is supposed to handle 25T of wrap. I will set the chain to fit the 26-52 combo with the RD fully extended, though I will never intentionally cross-chain anywhere near there. On the low end, the smallest combination I will intentionally run is 36-18, so my wrap requirement is (52+26) - (36+18) = 24. So, theoretically, the NR can do the job.
If there are any issues with wrap under the above assumptions, the long Rally style cage plates are a very tempting option.

Thanks again to Chas and all for the education on Simplex and other Euro derailleurs in the period that their losing battle for market share was underway.