So far from what I have read and seen we need at least:
Main action bar + extra bar at the same time
Warlocks and hunters would need that plus the pet bar.
Not sure if possessbar is only used if you are possessed or if you are possessing someone and gaining access to their abilities .. or both.
Override Bar from what I have seen adjusts the main bar depending on the situation at hand, usually in some sort of vehicle with abilities such as the argent tourney and the fairground games.
Looking at the blizzard code at :
http://wow.go-hero.net/framexml/16048
ExtraActionBarFrame contains 1 button ExtraActionButton1 - [extrabar]
OverrideActionBar contains 6 buttons OverrideActionBarButtonX - [overridebar]
The macro conditionals aren't inside the blizzard code ( I checked ) but they were mentioned by blizz on their forums as added and the overridebar one works and the extrabar one I had working but because it replaced the main bar it wasn't 100% correct so people wiped in dungeons due to no access to their main action bar at the same time.
nUI would work fine as it is if we can get the extra bar set up using the [extrabar] 13 setting to turn on its page but as an actual extra bar than replace the main bar.
As in when I had the following code set up people could see the extraactionbutton appear and use it but they couldn't fight as they had lost their main action bar buttons.
RegisterStateDriver(nUI_ActionBar, "page", "[vehicleui] 12; [extrabar] 13; [overridebar] 14; [bar:2] 2; [bar:3] 3; [bar:4] 4; [bar:5] 5; [bar:6] 6; [bonusbar:1] 7; [bonusbar:2] 8; [bonusbar:3] 9; [bonusbar:4] 10; [bonusbar:5] 11;1" );
hopefully that helps somewhat ..