So in total: you're starting with a non-flat plate, and want it to be flat in an unsupported, naturally sprung condition, after machining, of a grade with built-in stress?
Is the machining step going to thin and slice the material much, or is it more like a few holes, small slots, that sort of thing?
How flat is flat, anyway? If it's a few thou, it's not going to do it as-is just because of gravity. If it's by eye, you can probably do well enough bending it by hand.
Yeah, if you have a slip roller, a few back-and-forth passes, upside down and right side up, along both axes, should be able to do the job, a bit better and faster than "by hand".
Tim