'I would see no problem at all in using it in a specialised shanty session'

I can't think of any night I've ever been to, or any venue, where this would be acceptable.

In terms of, 'It is hardly ever, if at all, used offensively.' In this sentence you pivot from a historical framing ('many crews were multiracial') to assert that 'it *is* hardly ever' offensively used *now*.

If that's your intent, it's palpably and demonstrably incorrect. Everyone is aware of how loaded, how offensive this word is not only to black people, not only to non-white people in general, but to the vast majority of the British public whatever their racial heritage. And, in my experience, it is only used without strong declamatory framing by people who take either overt or inner delight at flouting these vital contemporary social norms.

People can furtively 'enjoy' whatever words they want in the privacy of their own homes, however revolting. People using them in public should brace for challenge. But, this sort of usage that seem seek to reserve the right to is clearly racist, whether or not there are others there to hear and object.