The alt gear ext on the B757 uses a small hyd power pack with its own electric motor. This runs for about 10 secs to unlock the gear and doors.
You would use this in flight if you either have a hyd sys failure, or cannot move the selector to down.
If you have a hyd sys failure, there will be no pressure to the truck tilt actuators. This would not be a problem. Truck tilt is only required before gear retraction.
If, on the other hand, you use alt ext with the hyds pressurised (because you cannot move the selector) then when the gear is extending, and the power pack turns itself off (on a px sw) then hyd would be available to the truck tilt.
Well thats the way I read the MM!