I opted to take the tree off above the cavity first. ran an eye bolt in it, cut it off at the bottom and had them use the liftall to set it on my trailer. Then I opened it at home with two cuts and removed the bees. It was a lot easier than working from a ladder, and the folks that were taking down the tree were mightily relieved to see the whole thing go away in one piece..
If taking the whole thing down is not an option, then your option is probably the best one available... It always terrifies me when running a chain saw into the cavity.. thinking of the bees on the receiving end of the blade.