Wiggy, it definitely seems like a customer option. Every Boeing I've ever worked on (737, 767, 777) has had a fixed chemical system, another poster pointed out that some have bottled.
In my outfit they are chemically generated.
I was under the impression that bottles were an add-on option for airlines who mainly fly through high terrain alts on a regular basis (i.e. QF 747s to UK) Seems I was wrong on that.
But it's definitely varying between operators because mine uses individual generators. They are also one of the largest 777 operators so this may have something to do with it