The idea got me thinking about these Booster gizmos again..
Flush Fit Register Booster Fan
But, what if you installed some duct fans in-line with the rooms
you wanted to control (adding more heating or cooling).?.
(Maybe with less blades, for easy flow-thru when off).
Connected those fans to a thermostat (or off-on switch) in each of those rooms..?.
Installing a few of these Booster fans would mean re-balancing the whole system,
with the fans off..
But, this seems like a low cost way of gettin er done..