The caliper adjusting spring is on the bike for a reason. I cured my brake squeal by:
A. Removing the old rusty spring, it had lost its correct springy tension and was stiff.
B. Ordering the correct part (45119-300-020) off
www.cmsnl.comC. Fitting the new spring and adjusting it correctly as per the shop manual.
If you have tried everything else it would probably be worthwhile. It's a very cheap and easy fix, well, it was for me anyway.
Good luck.