The Armour Archive

I'm contemplating it's use on mild steel helmets and grills. Does anyone have experience with it apart from gun applications?

No, but I'm suspicious. I don't think anything cerakote is designed to paint generally has metal moving against it, and I'm dead certain no part intended for it experiences significant flex.
Even a mild steel hat is going to flex whenever it gets hit. Even if it's a minimal amount it's still going to play havok on rigid finishes. I don't know if cerakote is rigid but it sure sounds like it.

Something on my long term agenda is to look more into epoxy coatings. I found out recently there are all sorts of epoxy paint which you can't find except from specialty suppliers (and they're all quite expensive).

If you're just doing bargrills and you want to stick with black (or want to put black anywhere else), then I advise looking into oil coating. It's essentially the same process as seasoning a cast iron skillet. I find that if you can start with a dark color (like leaving on the mill scale, or perhaps flaming it first) it's a lot easier to get a consistent black. If it's rusted and you knock off the surface rust and start with it browned, it's even better.

Bonus to that process is that if you clean your piece beforehand of any poisons, it's actually totally safe to do in a kitchen oven (you need a fan that vents outside, though).