The only other issue I can see with your setup, is that you may experience poor pressure control on the hydraulic circuit because of how it's plumbed (and essentially, that's exactly what the HPOP system is: a hydraulic system/circuit).
In your design, you're measuring the pressure at the same point [in the system] that you are controlling it (dumping it). Generally, it's best to measure your pressure at the point-of-use (injectors), and control it at the source (the pump). Pressure is measured in the head rail(s) (point of use), and control the pressure at the pump (the pressure source). This is why IH/Navistar originally designed the system the way they did...
The HPO rails in the heads are quite large, and act as a "reservoir" of high pressure oil for the injectors to use. The pump and IPR work together and are the source/control elements. The ICP sensor is measuring the pressure of these two "reservoirs" and controlling the pressure at the pump, via the IPR.
You will likely experience some erratic pressure control using the layout/design you are proposing.
I could be wrong, but 30 years of hydraulic systems work tells me otherwise.
Anyway, just my 2-cents...