In the thread, Carrier vs. Am. Std., Robotec said:
"If your supply registers are on the floor or low on the wall, then a ceiling return would be best."
I also have a lack of return upstairs, especially with doors closed. In two bedrooms, I have a wall which is shared with the stairswell. Has it ever been done where a return grill would be cut in on the wall at the ceiling in the bedroom, then a similar grill placed at a lower level on the stairwell side? These would be located between studs.
The location of the grills in the stairwell would favor the downstairs return grills, where there are 4, vs. 1 upstairs.