Walnut is supposed to bend very nicely. It is more brittle than oak though, so I'm not sure if I'll be reducing the frame's springiness.

I've bent (by hand, over a hot iron and also with a heat blanket) black walnut guitar sides and bindings cut from lumber supply kiln-dried boards and all the stuff I've had bent easily once it got to the 'hot and spongy' stage. Short grain areas fractured. Of course this was thin stock.
Air-dried black walnut - if you can get it - might be a bit tougher.
Definitely not a 'tough' wood like ash/oak/yellow cedar IME, so I'd expect the impact strength to be pretty low.