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The stock ECU mapping does maintain closed loop fueling even under part throttle acceleration up to about 4000 RPM, if I recall correctly. So as Tim points out, it isn't just at idle and constant RPM.
I think the key point to make is that the injector duty cycle is controlled by the ECU and is not *directly* a function of fuel pressure or AFR. What IDC to use at any given moment is calculated by the ECU using lookup maps, learned trim data, the MAF sensor, and in closed loop, the O2 sensor. Once the ECU determines the current state of things and has calulcated how long to open the injector for, it opens the injector. Fuel pressure is not accounted for by those lookups and thus changing fuel pressure has no direct effect on ECU IDC calculations, and thus no direct effect on the actual IDC.
Now, in closed loop, the ECU does reference the O2 sensor to see if the IDC it is using is generating the desired AFR. If it isn't, the ECU will adjust the fuel trims which then causes the calculated IDC to change. This only happens in closed loop, so an increase in fuel pressure can indirectly alter IDC in closed loop. However, the IDC would go down if the fuel pressure increased (due to overrun or whatever) since the ECU would see the rich condition and determine that the fuel injectors shouldn't stay open as long. So in closed loop, if we have fuel overrun, then the ECU will ultimately reduce IDC. And in reducing the IDC, removing the rich condition (where rich is simply richer than target, the target itself might be considered rich too).
In open loop, the ECU is not reading the O2 sensor and therefore does not realize that it is running rich (due to FPR overrun increasing pressure for example). Since the ECU does not know it is running rich, it will not alter IDC. So here we have the engine running rich and the IDC remaining the same.
It makes more sense to me now.
