I have been toying with this idea too… I would love to see a shape or surface that itself invites pushing over pulling and vice versa.
A flexible element is a funny idea. It can also be a simple rounded cylinder that sticks out from the door for the pull version, sits horizontal for the pushing version and vertically for sliding doors.
A few days ago I had the idea to create a conic element that can only be pushed yet still looking like a doorhandle. The pull version would then look like the continuation of the conic shape but with the sharp side into the door, signifying that it needs to be pulled, like a cork.
Another way, I wonder how that will work out, is to have a standard doorhandle that when turned, slightly moves forward or backward, giving the person the idea and feeling of where the door needs to go.
I like adding an icon or text too, that always works better.