Absolutly you can use it on hardened metals, it was hugely popular with the US army for gun parts, rifle bolts and recievers are about as hard as you'll find.
It give an excellent wear surface and is very oil friendly, hence the prefence for gunmakers, it was much superior to blueing.
That said, there are a lot of engine coatings check this article
www.hotrod.com/techarticles/engine/hrdp_0612_
engine_
coatings
please dont do anything that may affect the historical/resale value of the ariel. I have a soft spot for that stuff.
I'm going for a beer later with an engineer that designs heattreatment ovens for aerospace . He tells me the anti friction/ corrosion coatings do work.