That's annoying. Sounds like you've used up most/all of my ideas.
I've been dealing with loud mechanical disc brakes on my cross bike and haven't been able to get them to quiet down yet too. I tried sanding the rotors but didn't mess with the pads actually. Not a bad idea.
Someone told me to go to a big hill and repeatedly go down fast and brake hard. Then keep pedaling against the brakes to heat them up a lot. Claimed it would help. I've tried it a bit and it may have helped a bit. Hasn't made it go away for me yet though.