I'm fairly certain that I have used the
_root["main"+i]["subBut"+i] method to refer to 2 or more
dynamically created movieclips. I have been looking through my code
but I beleive that it is a work, so I can't verify just yet.
However, you might try this:
var tempMC:MovieClip = _root["main"+i];
tempMC["subBut"+i+"_"+t].label.text = "Test";
I know this is an added step but it might get you going.