Just based on the color rendering in the Moss catalog, those appear to be zinc plated or galvinized. You could try and see if the coating can be removed abrasively, make sure you mask up to do so. This is something you would need to do anyway in order to weld zinc coated steel. When welding even after zinc removal try to ventilate the welding area as best you can and wear a welding respirator while welding and until you either leave the area or the welding fumes have cleared out.
If it is not a coating on a substrate and the issue is the base metal itself, then you might consider the McMaster aligning weld nut 90955A113 if we are talking about the nut being 1/4-28 (SAE/UNF threaded). That's the best option I can think of off hand since my other "go to" places for odds and ends like that didn't show anything. The C1008/C1010 are closely related low carbon steels with good weldability. Just make sure to give things a good primer/finish application for corrosion resistance.
Bonding is another option, but I really don't like it if it is for a fastener that needs to be removed then reinstalled on a regular basis. I've seen too many bonded nut plates pop off on regularly serviced items to have really good warm fuzzies about bonding under those circumstances. That may be a non-issue depending on the specific application on your car Using a belt and suspenders approach, maybe bond and rivet?
Finally, perhaps try to get some from Rimmer Bros in the UK. In their listings for the CN2 and CN3 (elongated hole variant) cages, the renderings appear to have small bit of surface oxidation. That would be indicative of no plating. If push comes to shove you could always contact them and ask. If thier stuff is what you are looking for, it may be the option for you. I have had good expeience with them and Revington in the past on orders when I couldn't find what I wanted on this side of the water