Andy - I do not know what the symptoms are of premature B2 piston failure. No one has ever really said. Other listers here and another MB site say that the piston just fails and forward gears are inoperative.
It must be possible to "coax" the 124 tranny, as my mechanic did and Stu Ritter (I think he is the one who wrote about the faulty B2 piston) noted in his write up that it could be possible on the 124. I would think you would have to have a hoist to have easier access to the underside of the vehicle. I would not think sitting on jack stands would give you sufficient clearance since everything is so tight uner the 124. The 126 is a wider vehicle an allows easier access to the piston.
My mechnic said that the 190 is impossible to get to the B2 while the tranny is still in the car.