You could use the pump to charge an accumulator instead of direct off the pump
This peaked my interest, so I had to investigate deeper.
In addition to the accumulator idea, I also quickly revisited the possibility of overdriving the pump to get it up into it's intended 3600 rpm operating range, but quickly discovered that my best / cheapest bet is to just replace the pump with a much larger displacement, intended for 1500 - 2500 RPM max.
Looks like $300 will get me a 3.84 CI pump that will give ne 24 GPM at 1500 RPM.
Typical HP calculators say I need like 20 +HP out of the little 9HP DJ engine, but I'm betting the diesel torque will handle it with little difficulty.
As it is now if I bottom out the cyl. at 2750 psi and the engine at about 1200 rpm, the governor doesn't even move, nor does the engine even flinch, it just keeps idling.