I'll say this much...
I'd certainly like a pop-out Tag Window. I'm thinking, a mode-less window that you could position independently and resize (such-as, onto a secondary display) just like a Detached Display. If it had a detach button on the toolbar just next to that maximize button, and then (just like the Display AW) the AW area would show "click here to re-attach". That would be sweet!
But...
What exactly are you trying to accomplish? Adding tracks on-mass to a variety of different set-lists (then you open the individual lists and organize them as you'd like for playback at the individual events)? That task might be better suited to some panes, or some other method. The Playlists Field, which you're tagging in the Action Window, can be used just like any other field, and you can make it a pane in a View (and tag right from within the view where you pick your files). It might not help, but you can resize the height of the panes, of course.
If we can understand your work-flow a bit better, we might be able to help come up with some kind of solution.
At least, until we get a pop-out Tag Window.