I don't think any gain of performance obtained by sculpting the helmet better would be illicit. Actually I'm sure I've read/heard in one of the many Renault chat/podcasts that the helmet was part of the elements tested in the windtunnel, and now I remember they said Piquet did try different helmets (from different brands) to test their impact on aero performance, rather than searching more comfort. And this was during a track test. I was surprised because it involved that the difference was supposed to be perceptible enough to be recorded among general data from an on-track run. I guess they just measure amount of air entering the airbox at similar speed, with similar wind conditions...
Now for the fluid/gaz adding part, it's far fetched IMO. A fluid would be detecable, and before even taking it seriously, does that exist? I mean, is there a fluid or a gaz, that could improve engine efficiency while mixed in the air in very little quantity
(as obviously you wouldn't fit more than 50cl bottle in the helmet) without compromising reliability.Especially with a rev limit like now, it's unlikely.
Furthermore, it would be a very delicate operation to integrate the device in the helmet, because it's not manufactured by the team. And in case of accident, it could be revealed to everyone when it leaves the helmet
(makes me remember Massa lost the aero bit on top of his helmet during a GP last year). No team would take such risks for such marginal potential gains.
Last note. Why in the hell did you choose a pic of Lewis Hamilton to illustrate
a potential source of illicit gains? Now we'll have the usual trolls shouting everywhere that LH cheated with his helmet
(with the complicity of the machiavelic Ron Dennis)...