I've been doing some reading/reflecting on this today and I don't think it's the kava, actually. Since I started the topic I'll say I think this is coming from a candida overgrowth I've been experiencing. I've been on a paleo-ish diet for a few years now, but have cheated pretty hard for the last five months or so, and just recently got pretty sick for about two weeks. Illness can give way to a candida overgrowth if it's ready to do so, and I believe I was ripe for the pickin's, so to speak...
I also recently got folliculitis, which is a yeast infection of the hair follicles on the chest and shoulders (yeast, i.e. candida), so I think that's another sign that the metallic taste I've been experiencing is a result of that.
So, I'm going to work on correcting my diet, cut down on coffee and give yerba mate a real go, and see if I can't make an improvement.
Long story (not so short), I don't think it's the kava. I'd never experienced a metallic taste before with kava for the last year and a half I've been drinking it...