With the power liftgate you manually set the height you want and then press and hold the close button to lock it in for future use, so a little different than what was described. That said, I was under the impression with the manual liftgate it was either fully open or fully closed.
Shorter strut would do it, but the pressure rating may be different when holding the liftgate at a lower height. I imagine it is a bit heavier to hold in a partial open position than the fully open position. Probably can be done, but would need some effort to figure out how to get the right length and pressure/load rating on the strut. Might also be able to do it with strut limiting straps if you could figure a way to mount it. Either way requires a bit of engineering and/or fabrication.