What a fascinating discussion! Sorry I got into it so late in the day, but here are a few afterthoughts you might care to consider.
I was once priviliged to hear Willard White performing the role of Wotan in "Die Walkure". He did it superbly. Would Wagner have approved of an Afro-Caribbean singer playing his Nordic hero? I doubt it. Does that matter to me? Not at all.
I have also heard Dick van Dyke attenpting to sing in a Cockney accent in the movie "Mary Poppins". Despite his wildly inauthentic vowels, he made a pretty fair job of the role. As someone born and raised in London, do I resent this "appropriation" of "my culture"? No way.
Two of the world's great opera stars (Jose Carreras and Kiri Te Kanewa) have recorded songs from "West Side Story". Despite their awesome technical skills, the result is somehow lifeless. Is this just because neither of them are native New Yorkers? I don't think so. And as they were not New Yorkers, should they have been discouraged from even trying the experiment? Not in my opinion.
Ethnicity? Schmethnicity! The question is, does it work? And the only way to find out whether it works is to try. Musicians and singers - like painters and poets - are roving magpies, liable to pick up anything that glitters and use it if they can. What matters is not where they found it, but what they make of it.
Remember that Shakespeare stole most of his plots from other authors - and foreign ones at that. Does that make him any less of a great writer, or any less English? Of course not. And does Shakespeare's Englishness make it impossible for actors from other ethnic backgrounds to deliver authentic and moving performances of his plays? Perish the thought!
But what about traditional folk music? Isn't that more personal, more intimately associated with our sense of self - more likely to be defiled by the touch of an outsider? Well ...maybe... but when you look at them carefully, it seems that most traditional folk cultures have always been fairly promiscuous, gathering in songs and tunes from foreigners as readily as Shakespeare appropriated plots from other authors. Whatever works is retained, and eventually becomes "traditional", What doesn't seem to work (today) goes back on the compost heap, perhaps to be reborn in a different form for a later age.
Just to illustrate the point ... a recent publiation has pointed out that one dance tune in the repertoire of William Kimber, the great English concertina player and Morris dancer, was also recorded by Lead Belly, the great African American songster. Who got it from whom remains obscure, but that's what Uncle Pete calls "the folk process" - long may it flourish!