Yes, you can only be, "a member of crew" if you are on board a Air Work sortie.
For Airwork, your crew workers compensation must be covered by their employer, not necessaraly the helicopter owner / operator.
Charter, RPT, carries a mandatory Liability cover (limited) which i thought was 500K in this country, could be well wrong there.
The cover has grown, from 60K when I first started charter, especially since some clown drove an airplane full of ice watchers into Mt Erebus.
Just remember if you are Air Work, or private, your exposure is NOT limited. especially if negligance is proven, or even pursued it seems some times.
Some hirers demand specific amounts of 'seat cover'. Simple, arrange it and charge them as part of the service fee.
Make sure that any extra seat cover that you organise cannot be subrogated by the particular state's (in which you are operating) own workers comp policies. I.E. they can legally take yours to pay theirs.
Just try operating Charter without the pax insurance cover, and get sprung, and you will be fronting up with large amounts of cash that involve many noughts, and that is even before they start askimng questions about the legality of your ops' etc.
I haven't resourced whether these arrangements date back to the old bible, the 'Chaicago Convention'.
most other things in aviation do.