How to accept drag on Dock expose window?

Question

When you drag to a dock icon and pause, all of that app's windows are brought to the front in an expose-like view. Dragging to those windows will outline the window in blue, but how do I get the window to actually accept the drop? What is being queried during this expose process?

I had thought that maybe it would be treated as a drop onto the NSWindow itself but this doesn't seem to be the case. (I can successfully drag and drop onto my windows when they are normally visible.)