I've read everything I can find on the subject of flushing tranny and torque converter, both pro and DIY.

Probably a question for Gadget (studied his write-ups intently) but why can't one hook up one's own resovoir, simulating the "pan", thus making a pump inlet flush machine? What else does the pro thing do that this wouldn't?

I haven't been in an auto tranny in many, many years so forgot what they look like - what's in the pan to supply the pump? If it's some kind of tube couldn't that be extended into a 5 gal bucket filled with 4 gallons of ATF?

I was planning to do "fill the pan, let it run through, shut down, refill, etc" but seems it would go a whole lot faster to do it in one pass.

I haven't seen one of the flush units in action, so I can't comment on how they work, but trying to simulate the 'pan' with one's own reservoir is not as easy as it sounds. The pump in the trans pulls the oil up through a port at the bottom of the valve body which is covered by the filter assy. The fluid level in the pan is supposed to be high enough to ensure that no air is drawn into the pump at any time. If air did get in, it would cause the pump to whip the air into the fluid and make it foam. So you would have to ensure an air tight fluid path to the pump, and I'm not sure if it would be possible. Then you have to collect the 'old' fluid someplace. And I assume you intend to run the engine in order for the trans pump to function. Sounds like it could get pretty messy to me, especially if you have to do this on the 'ground'. The shops that advertise trans flushing in my area generally get about $125. for the job. To me, not too bad considering the effort needed to do it yourself.