Can you explain the flow loop you are thinking of? Pump, reservoir, rack, back to pump?
Most systems the reservoir is to keep extra fluid for expansion and leaks. also, burping air from the system and cooling by the extra volume. I dunno how your set up would work till you give more info to the system. But it would have to be a good tank to deal with the pressure.
Good overview:
http://auto.howstuffworks.com/steering.htm