>>I have 2 21" screens here and MoI is limited to only one of them. Modo, Photoshop, etc, let me drag panels out of
>>the main interface and onto the other screen so they don't clutter the main window.

For two monitor people here is a possibility.

There is the optional left side panel which possibly extends the full depth of the MoI window. When the MoI window is extended partially onto the left hand monitor, that left panel will be on the left monitor.

I'm really thinking of the way the UI works in Blender...any arrangement of multiple windows can be set up within the main UI which can then be stretched over multiple displays. So the UI can be simple or busy as you like.

>>It may be possible in the future for me to focus on something more limited than that, like only being
>>able to tear off large panels that take up a whole edge of something rather than trying to stack small
>>pieces together. But even that will probably be a ways out.

May be worth taking a look at the Blender UI.