I slapped together the necessary components to make the RFID reader work with the MCU and toggle the relays, wrote the code, fired it up and...IT WORKS!
I've decided to add a third RFID tag that can be placed somewhere on Thor's costume so that it can be read by the hammer and re-magnetize without being too obvious. The other 2 RFID tags in his boots will demagnetize the hammer, but there wasn't a good way to turn the magnets back on. This additional tag takes care of that.
The hammer's handle (aluminum rod) and main chassis (aluminum plate) are being welded together now, so I can start kitting it as soon as I get that back.
I still have to move the electronics to something a bit more permanent than a breadboard, but otherwise this project is progressing nicely. My primary concern at this point is the 3D Printing. The hammer is massive and has intricate detail. I'll have to spend some time figuring out how to make that work.