Seems unnatural and the truck would never see spring compression like that in use.
But supporting it with a non-stock axle should be fine.
It is exactly the same spring compression as if it were on the ground sitting on its tires. The spring compression is the weight of the truck above them.
I will go back and forth between lifting by the frame vs the axles depending on what I am trying to do. In this case I specifically wanted the suspension at ride height to set the routing between the axle and the frame for the neutral postion, then switch to lifting by the frame to check it at full extension.
