Mark, Why not try a hair dryer, and get that plastic really warm and slightly pliable?
Joel

I started with the hair drier, that didn't seem to do much. Part of the reason for going to steam. In the last try with steam, I could 'see' the part heat up and bend further back. I clamped it flat but after it cooled, it went right back to the old bend. I suspect what I saw were the residual stresses from the original molding were relaxing.

At this point it doesn't sound too promising. From the picture you posted, it's a nasty warp for sure.
Joel