You can do certain % of boss encounters in tk/ssc/hyjal and bt. Some encounters will be harder then it would be without boxer, and some impossible.

For example, hyjal.

Winterchill
You will get alot of damage from death'n'decay first of all. Tho you can survive and it isnt that huge problem. Also you will get ice bolts and wont be able to react fast enough with bg trinket and more likely get 1-shotted. Not sure if grounding totem absorbes ice bolt.

Anatheron
You will have to place your characters in different places of the map and manually control each toon once it gets infernal. Pretty hard but should be possible.

Kaz'rogal
More likely you will have to do the same - place toons around the boss so they wont blow each other.

Azgalor
1 rain of fire will affect 4 toons. Uh, hard work for healers. Also rain of fire and in the same time one of you gets mark of doom. Someone gotta die for sure

Archimonde
It will get alot of work. You will have to deal with air burst, fear into fires and fires themselves. Gotta be very tough.


So it is possible to raid hyjal and bt but will be very hard and high end guild will prefer different people instead of boxer.