I very much doubt that effective 2 way hydraulic damping could be successfully applied to radiadraulic forks. Rebound damping is easy to obtain as the oil is contained in the bottom of the forks, and the damping is controlled by regulating it's escape past the piston - using the hydraulic principal that liquids cannot be compressed. Damping in the opposite direction could only be obtained by generating a partial vacuum below the piston, so the amount of damping generated would be limited by atmospheric pressure bearing down on the top of the piston - at that point a void would be generated below the piston, resulting in a sudden shock load as the suspension recovered. Also, when damping is increased, spring rates often need adjusting, otherwise, on a series of bumps, the suspension may not fully recover before the next bump is hit - resulting in the suspension pumping itself down onto the bump stops!
When I was researching the 1950 ISDT machines, I was loaned some ex-factory documents that included some details of the experiments that Eddie Withers had carried out. There was mention of 2 way damping but no actual details - so I guess the experiments were not successful enough to warrant the modification being done to production machines.
Regards,
Eddie.