No, that won't work. It has to be fed through the fill case. That is why it helps to have a spotter so you only drain a couple quarts at a time. Then fill two quarts and repeat until you have bright red fluid. As I recall, it only takes about 10 seconds to drain a couple quarts. Using a long hose to the fill case makes it a simple process. I add a small amount extra from what I drain, get to temp and pull the overflow. Pretty easy.
Yeah you are correct. I was thinking of how this works:
BG PF5 Power Flush and Fluid Exchange System | BG Products, Inc.
The BG Flush machine taps your line and it pumps new fluid into the return line. In theory the above video leaves a small amount of original fluid in the system (whatever was in the return line that never gets circulated).