A possible solution, given that we're talking about pretty advanced bioengineering here. Why not just go whole hog and get rid of the heart entirely?
Basically, the heart is a kind of invagination of the great vessels with powerful musculature and some electric stimulation to keep it ticking regular.
Rather than a single, central pump with a sub-station pump in the head, engineer a system where all the vessels (from the aorta ~ vena cava on down to the main branches and lesser vessels) pump peristaltically? Combined with a system of back-flow reflux valves (such as already exist in the veins) you should be able to achieve a continuous & active flow of blood throughout the system.
I imagine that the check valves, especially those in the head and neck will help reduce or eliminate the effects of gravity 'sucking' the blood away from the brain. Also, the peristalsis will keep the blood flowing continuously and thus perfuse the brain.
Obviously the bioengineers would be using a type of cardiac muscle that, like the muscles of the heart, do not tire the way skeletal muscle does. Just like a heart, it won't do for the system to rest or shut down untimely!
The long and short of this system is that you have "lots of little tiny hearts" in the brain (and everywhere else)!