I picked up some of these at lunch today at H.D. I think they will fill the bill nicely (if not perfectly) for keeping the headboard from hitting the wall and doing damage. The diameter looks to be less than the size of each corner, so they should be nearly invisible. They have a self adhesive backing. Directions look interesting. They say to apply to the wall (as they are door-handle bumpers) and hold for 3 minutes. I wonder what is so special about three minutes. (Edit: I intend to mount to the frame corner, not the wall. Though now that I think about it as I type this, that might cause hemispherical depressions in the wall.)
Once she is out of decorating mode, I hope to mount it to the wall if she will allow it.
Thanks everyone for your great ideas.