In my 100+ year old 2-story house, it's quite warm upstairs. One bedroom has a duct blowing in up from the air handler in the basement, and a return on the opposite side of the room. The second bedroom has a duct blowing in, but no return. The upstairs bathroom across the hall has a return, but no duct blowing in. There is a third room some 15 ft. down the hall from the bedrooms, 10 ft. down the hall from the bathroom, which is open on one side as a loft above a downstairs room.
Neither bedroom upstairs has sufficient airflow. We've tried closing all the vents downstairs, but this didn't help much. I don't believe it's reasonably possible to run additional duct work. The second bedroom I'm assuming is a lost cause. Fortunately, it's the guest room and isn't regularly occupied. The first bedroom however, we need to fix.
As sort of a bandaid on the situation, I'm considering building some small wooden boxes with fans which will slide into the duct work, replacing the existing vents. I plan to suck more air upstairs this way. If this isn't sufficient, I plan to build some into the return vents as well, blowing back down into the duct.
Other than that, "this may not work", do you see any issues with trying? I'm also open to other crafty ideas that don't involve ripping out walls at this time. :-) Thanks.