[Richard]

The beauty of the accumulator is the marriage of the compressible gas, N2, and the incompressible hydraulic fluid.

The precharge with N2 on the bladder is only a starting point.
The real charge is set when the closed hydraulic system is pressurized. In most cases that is done with a hyd pump, however in this case something like an external pump must be used to raise the pressure inside the closed system.

The magic that happens is as the bladder is compressed by the hydraulic fluid, the N2 inside the bladder increases to match the hyd pressure. Yeah yeah, you have said this about a zillion times. But, if there were no bladder, given the incompressibility of hyd fluid, then 1 ml of fluid added to a closed system could raise the pressure by a 1000 psi or more, another drop would add another 1000 psi. Now add the bladder with the compressible gas, and it takes a significant volume of additional hyd fluid to raise the pressure in system.

Why walk you through that? In designing and using the makeshift hyd charging setup, a few drops of hyd leaking will NOT have a significant impact on the pressure within the closed loop.

If I were doing this, I would install a cross below the accumulator. Obviously the vertical ports on the cross connect the accumulator and the system. On one of the other ports install a quality gauge. On the last open port install a hydraulic 1/4 turn valve. Now using the HF pump adapted to the 1/4 turn, pump up the system to the desired pressure. Close the valve, relieve the pressure at the HF pump and remove it. For grins, put a plug into the open end of the 1/4 turn as an insurance policy. If you didn’t want to use a 1/4 turn, you could use a check valve.