ok, I can see the pit falls, if not a factory fit job.
However…
Is there a law or regulation in the UK or indeed in any other country that says you cannot mount devices inside your lid?
By modifying your lid you may change its conformance with type approval for impact protection (such as famously, when Barry Sheene drilled a hole in the chin bar of his helmet so he could have a fag on the grid). You may then essentially be riding without a legal helmet, which you can’t do unless you’re a Sikh.
But you don’t do it, because it’s a dumb idea.
Agreed.
But what if a device could be securely mounted inside, so that the risk of damage was greatly reduced. For example, much like the scala rider mic kits are, but with a portion entering the face area.
Would that break any “rules”? - if it could be done safely.
Why would you want something hard inside your helmet (no pun intended) , as it is nice and soft currently. Imagine the worst case scenario…quietly minding your own business
going along at 50 when some idiot pulls out and clips you. Now you are tumbling down the road with that hard thing smashing into your skull every rotation!
Do you want ‘telluB’ imprinted on skull :crying:
just mount everything on the outside 
I have scala rider g9 mounted on a side and got small mic velcro’ed to the front of the inside chin (in front of my mouth). I also taped all wires with ductape so nothing is loose inside