Agreed about not modifying the helmet but I could imagine a helmet with such a thing built in.

Where's my inbuilt headcam already?

Honestly, first motorcycle helmet with inbuilt front and back cams that is at a vaguely mainstream price (premium allowed for extra features!) has my serious interest.

Built-in seems worse? I'd rather it sit outside on something flimsy and break away at impact.

Helmets are tested in a variety of impact scenarios without modification. Who knows how some random attachment might negatively affect those outcomes. Presumably you might use some sort of adhesive to attach things — that adhesive can damage and weaken the shell of the helmet.

I just use my brain and assume that the plastic 'candy' shell on the helmet doesn't do anything (other than moisture protection) and as long as the styrofoam innards are in-tact, then nothing materially changes.

Otherwise we'd also see warnings against stickers and artwork on the shell of helmets, but we don't. Or at least I didn't, but I don't really read manuals.

Every motorcycle helmet manual I’ve read explicitly says not to put stickers on the helmet. Protecting the foam from moisture is important — a rainy ride can significantly weaken the foam. Eg: https://www.bellhelmets.com/on/demandware.static/-/Sites-bel...

Trick is to have the light held with some kind of rubber band type thingy, so that in an impact, it just breaks off sacrificially. But I guess, in the unlikely event, if the impact is at just the wrong angle, it just drives through the impact-absorber. But I don't think it's possible to fall straight down (negative Y speed, but no X/Z speed vector) when travelling at speed.

At least that's how my bicycle helmet light works.