If the plating was done for cosmetic effects, or to prevent charring from nozzle blast or ejection charges I would agree with you.
Also agree with Alex that plating won’t provide much structural integrity.
But Recon says his purpose is “durability”. Since the piece is already plastic, we aren’t talking waterproofing. Unless the nickel is for UV resistance, basically sounds like what he wants is to “armor” the payload section. To me, that is using metal for STRUCTURAL support of the body of the rocket. If he DID put enough metal on to provide such strength (is this even possible with electroplating?), THEN I think it would be pushing the regs a bit.
That said, on long gap stage rockets I put a segment of rolled up aluminum can glued inside the chimney/stuffer tube just forward of the booster engine to prevent charring. So I am not exactly a purist either.