I haven't come across any one mentioning that the sprag clutch has gone on either 722.3 or 722.4 or 722.5 before - Mercedes usually call it the freewheel.
You can see in this picture when it is used (it is the column headed "F")=>
I think that it was most probably working if you had reverse.
The usual culprit for "no forward gears (well it is the 1st three actually) but still have reverse" is the B2 piston.
I encourage you to have a look at the transmission yourself!
722.303 in bits photo shoot