This question is based purely on old rumours, but was it not the case that the 737 was originally designed not to have hydraulically powered flight controls at all?
I believe that the hydraulics were originally only installed for rudder, spoiler and undercarriage actuation, with elevator and aileron power derived from servo tabs, a la 707.
The design solution of having the PFCU's either in the wheel well for aileron and aft fuse for elevator would seem to suggest a retrofit of the system, when the control loads were found to be a 'little' heavy.
Every other type with power controls has the actuators in the wing or stab itself...or am I totally wrong?