I like this idea too - however after implementing and experimenting with it, it doesn't seem to be that practical. Regardless of where the hot-spot is located, whether it is in a corner or on the side, or how large or small it is, it always prevents you from accessing part of the MDI area, which in turn prevents you from accessing a window's system menu, editbox, titlebar, sizing borders, or mark/copy of text in some situations.
There is only one situation where it does work: when MDI windows are maximized, the Treebar is on the left, and the hot-spot is in the top-left corner of the MDI window. It wouldn't really make sense to provide this feature just for this one particular display configuration.
Instead of using a hot-spot, another option could be to allow you to pop up the Treebar in an "On Top" state using Control+T. Would that be useful?
