Dunno about the bigsby thing, but I personally do not like brass for anything other than trem blocks these days, well if bone is available as an alternative anyway and especially on the nut. Thing is that brass is too heavy and compact, it can actually choke things a bit or so I (and a former Gibson masterbuilder I spoke to) feel at least. When I tried brass it made the sound more focused and warm, but also took shimmer, volume and life out. Since the nut only affects open strings this is especially bad, as those are after all the lowest postions of the strings, so having something that will keep em open, bright and clear seems good to me.
The tusk material seems good to me so far, but if bone is offered it just rules in my mind, so I would go with that. Or earvana, heard good things about it and Jonathan (Felineguitars) uses it and I trust that man.