I have done the trans flush on disconnected cooler lines, and I had no spills, what I did is have a milk gallon bottle, disconnected the return line and stuck clear hoses on both the feed and return lines and stuck then into the milk bottle about a 1/3 way from the top. The tramy pump did not pump as hard but since I had the feed line stuck in the bottle could never overfill, not that I wanted to have the feed line pick up old fluid, but more like a safety measure so that I could avoid the spilled mess.
As for the PS system, yes it is a hydraulic system and thus I knew that I was going to be dealing with high pressures, so I will be doing the procedure as outlined before and if I need the assist of the pump, I will be using and external electrical engine just to have the situation under control. Sure it takes more work, but at least it will be done nice and slow.