There is nothing that says you can't buy the helmet alone. It has an individual price (and isn't factored into the Full Milspec/Security Armor), which obviously shows it can be acquired by itself.
It makes sense, really. The helmet is the most probable piece of armor you can lose, so making them an individual piece you can replace is a good idea. If you're rich enough, you could have multiple helmets with different enhancements (or other toys, perhaps a Flask-Pack to disorient someone in a pinch?) in them for various situations. It is also a surcharge on top of the nuyen you need to put a Chemical Seal into said armor.
That said, I wouldn't let the helmet give Hardened Armor when not worn with another set of Hardened Armor. It'd still be a +3 Helmet with plenty of Capacity, which makes it nice, but not 'my helmet protects me from all scrapes and abrasions!' As far as Gear Access though, I'd say sure... If you can find items generally stored on a helmet. You ain't strapping a magazine to your cheek. Maybe some entrepreneurial 'runner or merc keeps his business card in a strap on his helmet, instead of the usual card from the classic deck?
P.S. Obviously it is all the soft-side velcro, covering the jaw, cheeks, chin and upper lip. The helmet is so manly it has a beard, no exceptions.