Have you ever considered a "taste test" for zinc and potassium?
While it is a long shot, we do hear from a few people in the heavy metal detox groups who had a metallic taste in their mouth in the days after improper silver-mercury dental filling removal.
I think the theory is that mercury is somewhat similar to zinc--cf periodic table. However Hg is the big bully and displaces zinc to some extent.
If potassium is involved, you'd probably want to look into adrenal issues such as aldosterone.
Again, it is a long shot but better than no leads at all.