..Quick question...
..can carbon-boron infusion (from Attitude) be added to Helmet/ballistic mask?

Is there anything in the rules texts that lead one to the conclusion that it can't?

Attitude tells you in the sidebar on p. 160:


So, by RAW, you're limited to either generic clothing AND the very specific armor clothing.
You could also allow to add it to other types of clothing, but personally I'd recommend refraining from allowing it for actual armor. It's too easy already to stack armor sky-high.

The only limitation is that those mods can only be added to clothing (armoured or otherwise). If your GM says a helmet counts as clothing, then go for it. Yay for vaguely worded rules!

..so forget about helmet (wear ballistic mask & Death mask), just buy shirt/underwear with carbon-boron, wear it under regular armors to maximize protection..

Underpants Line:

Jewels by Gladio
"For Their Protection"

Actually, in a shocking turn of events, Hardy corrected that here. (i will give credit where it is due, even if there could be more credit in the bank )

Tldr; works on arsenal clothes, treatments mutually incompatable, count as ratings for armor mod limits.

That's the same Jason M. Hardy who said that karma gen should have 1000 karma to reflect the 'changes' made in SR4A, right?

Jokes aside: Yes, that's probably as it was intended and how most people (including me) play it, so it's nice to see that confirmed.
On the other hand, formal credit would only then be due if it was published in an official and downloadable FAQ document, instead of in one posting on one forum. At least they seem to be on the ball with that for SR5 now.