Hi TB! I know I get a metallic taste in my mouth sometimes when my illness acts up. Probably not as bad as what you are describing but maybe keeping some strong tasting candies to chew on may help ease the taste buds some.
I have to take alot of pills everyday for my illness and every now and then I have problems getting them to go down and they instantly melt in my mouth before I can swallow! Yuck yuck I say! LOL! Bad thing is that it lingers and taste something nasty. Use everything in the book to get rid of the taste and nothing works. I now have a stock pile of cough drops and that seems to help ease the rotten bitter taste.
I am so sorry you are having to go through all this and do hope the IVIG helps you get stronger and well. So I will keep you in my thoughts and prayers! Hugs