Transmission drain/fill question for those that have done it

Question for anyone that has done a complete drain/fill of a ‘sealed’ transmission: since it is common practice to use the trans.oil cooler line to remove fluid by letting the transmission pump it out, is there a reason to not use the return line (from the cooler, back to the transmission) as a way to replace the fluid? I recognize that the transmission only ‘pushes’ fluid, and does not ‘pull’ it through the return line.
What I’m thinking is to use a large (the full 11 litres) container of new fluid sitting up higher than the transmission, gravity feeding into the return line back to the transmission, while the outgoing line is pumping out the old fluid.
Obviously you’d want to monitor the rate of both new fluid in and old fluid out, to be sure they are moving at about the same rate. If gravity feeding would not keep up to the pump, it would be simple to use a very slightly pressurized holding vessel, to push the fluid going in at the same rate it is coming out.
Has anyone done it this way? Is there some reason it could not/should not be done this way?
TIA for any advice or comments