Just some notes / ideas.
I don't know how you're implementing the group hotbars.
I purposed long time ago to have them respond to click event by two ways.
First, /targetdefensive <member_name> (since it's a list you cannot use /targetgroupmember #). I don't know if it would be possible to do the member_name thing (it's a variable extracted from the actual list entry, isn't it?)
Second, useability or button press or whatever is needed to send in hotbars (cannot remember).
Another idea I really love. I modded Drox's Off. Target window to have a small hotbar bellow. Obviously it's inside the target's window so only displays when you have a target. This keeps UI clean when not in combat but also gives you some buttons to click when you are in combat. It's pretty nice and clean, and having most commonly used abilities just below target's health is a great way to keep eyes in important things.
If it is possible to do the group thing, it would be also posible to have a hotbar for the encounter entries.
Imagine you pop up a Options window for setting up your UI.
One of the options are "Group and Target hotbars" and you have there a 5 slot hotbar (name it hotbar 101) for group, a 6 slot hotbar(name it hotbar 102) for Off. Target and a 2 slot hotbar (103) for encounter. Those are drop targets for abilities / macros / items.
Then on each entry you create a hotbar and use the corresponding slot (102 101 103 etc) but no drop target not drop source. Also, the events of those objects are dispatched to two listeners, fist the /targetXXXX <name> and then the action itself.
That way, I can as paladin (for example) place a taunt and a root on Encounter, taunts on target window and heals / rescues on group window. I don't have to worry to press'n move the mouse (I won't move the icons, they're not dragsource) in the UI, but they're in the "Options".
When I need to do it, I taunt any mob (and it becomes my target). When I need to heal / rescue, I do it with only one group button.
But it's important to keep them no "drag" source, so you can have smaller icons (30x30 works for me) and had no risk to move them arround the screen instead of clicking them.
Ju8st my 2cents brainstorm.
Will organize this in my mind later!