AFAICT the approach of using a "keep out" works when you know in advance of placing the power plane that you want to make a hole in it, and where.
All that seems to happen if you place a keep out after the event is that you get a DRC telling you that the keep out has stuff in it that shouldn't be there.
So the track and delete island seems the way to go.
Unless of course ...
why dont you use the poligon splitter option of UB. You can splitt the plane and delete the unneeded part of the plane.
Greetings from germany
I've confirmed that Garret's method works wonderfully, and is less complicated than the trace and remove island method. The main thing is that the keep-out layer will make the power plane not draw there, but will not remove anything else. 🙂