If the electronics options (lots of techno mumbo-jumbo) is not in your comfort zone I'd think there are some mechanisms out there...most likely from the automotive world...that should be adaptable. For example, it seems a similar mechanism is the motorized-hinged tail-gate lift on a mini-van...or the motorized-hinged gull-wing doors on fancy cars. Varying torque is required to lift the gate or door (at constant speed) depending on angle. Likewise, varying torque (and less of it thanks to gravity) is required to lower the gate or door (at constant speed) depending on the angle. Obviously, in the automotive mechanism, the use of heavy counter-weights is not an option since that would add to vehicle weight. I can imagine these might be an application for linear-actuator motors as suggested earlier. In other words this is option 1 of finding a different motor gear/transmission that is stiffer in that speed variations are demoted in the presence of varying load torque.
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