You could use a set focus-round robin button?

e.g.

Main presses 'a', on main this does:
/targetenemy
it also sends 'b' to window1, which does:
/focus maintarget

Main presses 'a' again. this does:
/targetenemy
Also send 'b' to window2, which does:
/focus maintarget

Repeat 5 times, all 5 shamans have a different focus target.

Then main presses 'C' which gets sent to all windows and does:
/cast [target=focus] Hex

... main spams 'a' 5 times, then presses 'c' and voila 5 different targets hexed?
I'm at work so i can't test this, but it should work?