Rick,

In my opinion; no, there is no way to rid yourself -reliably- of bow and or
twist on an existing PBGA.  It would seem to me it is a standard warpage
issue that is probably rooted in an unbalanced design, not so much material.
A pretty broad statement but I think true.
If you are successful in straightening out the curve, I think it will come
back to haunt you.... more sooner than later.

Coretec Brad