I am using jQuery droppable (in conjunction with jQuery draggable) to allow the user to add rows to an HTML table by dragging items from a list and dropping them on the table.>
This is a rather crude (and codeless) solution, but you could try using the hoverClass option with your Droppable and creating a new class called "hovering" to set Droppable behavior that only happens when the Draggable is hovering over the Droppable. This "hovering" class could then (this is the crude bit) run some sort of endless loop or some other sort of checker; I haven't used these classes before, so I can't think of any more specifics past this point. =/
Edit: Even cruder, you could alternate disabling and enabling the Droppable using the "hovering" class; I would definitely do this synchronously though, and with a generous time delineation as well. The alternating disable and enable calls should trigger one of the events, though which one you'll have to experiment with to find out.