What about attaching the stretcher to a solid sheet of plywood ?
then re-framing to hide everything ? To save ruining the canvas ?
John T.

That may work. IF the plywood were thick enough. A piece of plywood
40X30 would add weight; frame + stretcher + plywood. Thinking: I wonder
how thick the plywood would have to be to be strong enough to pull the
bend out of the stretcher.


A rough test - see what muscle it takes to bend a 5/16 piece ?
or a 1/4 with a rigid X-brace ? just random thoughts.
I would be worried about damaging the canvas - by replacing the
stretcher ..
Also - perhaps an old experienced picture-framing place could
offer some advice - or give you a quote on a Pro Job .
John T.