Hydroboost w/o power steering..possible?

Pressure is a result of flow meeting restriction. Without having the definitive answer, I would conjecture the "restriction" is the valve in the PS box, and the hydroboost is flowthrough, in-line with the resultant pressure generated by the upstream pump meeting the downstream PS valves. The valve bypass freeflows back to the pump in the low pressure line. Think of it this way, if there were no PS box and the pressure line dumped back into the reservoir, the line presswure would be essentially zero, but flow would be at its max. The Hydroboost does not generate a restriction on its own, it just flows whatever pressure is present in the line through, and if it needs some, it diverts it, uses it, and has its own low pressure return.

This being said, a hydraulic pressure regulator downstream of the hydroboost unit would build the necessary pressure and bypass the excess flow at low pressure.