I think this is a good start. I wouldn't do the hover/right click thing nor the scroll bar. There's no time to sit quietly in RTS. U need fast and synthethic information to make fast desitions
Just show the units this way (maybe type-unit in the first line, and then just the health bars in the lower lines as said before. The settings in the options menu could be used to apply this filter. And u could click the bar to ungarrison the group of "-%" or "+%" previously defined.
At least, hotkey implementation using actual "Wounded unit health" its a simple good start.
And I want to add, regarding your topic the fact that Control groups cannot be selected when garrisoned, I think it should be able to keep selecting them even if they are garrisoned. Maybe this option actually exists and i'm not aware