Now that's a serious pump. How much drop are you looking at from the drains to all of the reactors you are planning, I'd be concerned without enough pressure (assuming gravity fed drain so 1G x height) they'd be apt to stall on you. I would definately feed each return line directly from the manifold, so each will be able to carry the full capacity of water possible and reduce the back pressure, might consider stepping up to 1" lines from the manifold even if it's tapped at 3/4", the extra capacity will improve the overall efficiency.

Metering the flow to a given device may prove difficult and expensive, especially given there shouldn't be any metal contact for something like a flow meter. Not sure a pressure gauge would be very useful here either....

We'll let some more folks try to wrap their heads around this one and see what they can up with.