Peter - It certainly could be done, but I expect things like expense are the determining factors in the choice for gear pumps for implements. Another reason may be that most implements and cylinders are rated not much higher than the relief valve setting, so increased pressures over what a gear pump can generate aren't practical, anyway. Besides, it's the flow, not the pressure that's of primary importance in powering most implements - this is certainly not the case with cylinders, of course, but, as I said, the gear pump produces all the pressure they can handle anyway.
You bring up a very interesting application, as it relates to HST transmissions, though. In the HST transmissions that most (actually, all, to my knowledge) CUT's use, a variable displacement pump is mated to a fixed displacement motor, with the displacement of the pump controlled solely by the operator of the tractor, via the HST pedal.
With the EarthForce machines, and most larger HST powered equipment, I've found out, they use a design called "automotive control", which incorporates a variable displacement pump and a variable displacement motor, with the displacement of the pump controlled by a combination of operator input, via an "accelerator" pedal, and backpressure from the motor. This "accelerator" also may or may not also control engine speed, depending on where the hand throttle is set, i.e. if the throttle is set at 2,400 rpm, the "accelerator" has no effect on engine rpm until it's depressed almost all the way down. Thus, you can set the engine rpm high enough to supply whatever hydraulics you need to the cylinders and/or implements, no matter what your ground speed is. The motor's displacement is controlled by internal pressure.
It all sounds pretty complicated, but the net result is a pretty versatile and easy to use system. For example, if you start out with the engine at idle and the tractor motionless, and floor the accelerator, the tractor will start out slowly, with both the pump at or near minimum displacement and the motor at or near maximum displacement and, as its speed builds, the pump gradually moves to maximum displacement while the motor moves to minimum displacement. At this point, highest ground speed, and lowest "power" (torque to the wheels) is being produced. If load increases, as in starting to climb a steep hill, the motor will increase its displacement, reducing its speed and increasing available torque, as its internal pressure builds, and the pump will also adjust to match the "accelerator" position and the load of the pump.
So, as you can see, one advantage of the addition of a variable motor to the system is the effect of an "automatic" range selector. It provides greater torque than a typical fixed motor, because it can have greater displacement in it's maximum displacement position than a properly sized fixed displacement one. On the other side, it provides much higher speeds (at lower torque, of course) at its minimum displacement position. It's a slick system, but a lot more expensive, too, of course.
MarkC
