Seems like Brazing would be my first choice for a Cast Iron plate.  Welding to my understanding is not easy on cast iron.  Since it is going to be plated, I wouldn't worry about matching the material and buying a new casting saves a lot of effort.   Might consider Solder on the Brass to be plated one.  A proper joint wouldn't require a Blind Screw for strength.  The plain Brass Plate becomes a challenge as you're dealing with 3 different alloys and the possibility of different colors.  I've seen a Scanner that can identify Alloys but they were Big Bucks!  An up to date Scrap Yard may have one and a Box of Donuts works wonders!  More coffee 🤔  David

I have a picture somewhere of my butchered Hepburn buttplate. Mine was cut off and then heavily shaped and contoured with a file, one of the harder ones to fix in my mind. 

The match A Hepburn on gunbroker appears to have had the plate lopped off with a chopsaw leaving it smooth and flat and Much easier to mate up with two donor prongs. Bob, your article in Journal also got me thinking again about this.

Ultimately, even an original solid brass buttplate once fixed would be nickel plated.  Not ‘original’  but at least saved.

This is all assuming that nickel plating can evenly plate over differing metals agreeable to the process.

I would agree, the gunbroker plate would be a good candidate to try repairing.  I would think tongue and grove it and then depending on the fit either silver solder or braze it.  No screw would be necessary then.  Brass would be easier to shape compared to the steel plates I have made.   

Be a good question for the nickel plater on how different metals work with the plating.

Be a good winter project for you!
Bob

Jordan, I had to do this once with a rare Model 1881 Marlin that had a factory small Farrow buttplate that some moron chopped the prongs off of. It was brass, and no nickel left, so I found a reproduction brass buttplate I planned to put on the 1881, but when I tried it the buttplate was smaller than the stock, and I didn't want to work the stock down to fit. 
I left it alone awhile, and then thought maybe I could cut the prongs off the donor repro and fit them to the original buttplate. So I began by cutting them a little long, and then I ground the prongs off at 45 degrees where they met the original plate. I marked centers on the buttplate and the prongs and drilled and tapped for a 10-32 thread. Used some 10-32 all thread and made up studs to screw into the buttplate and the prongs. I screwed them together, and removed a small amount of material off the prongs until they indexed up correctly when tightened. Then I fired up my oxy/acetylene torch and brazed the prongs on to fill the area ground off. 
After that it was just filing and polishing the prongs down and they were done. Never got it nickeled as I just left it bare as it came to me, but I'm sure it would have nickeled just fine.