Well, I think you could probably use the offset= parameter to do that, but you'd have to set it for each submenu based on the height of the submenu off which it opened, and what happens if someone has really big text set? I think that would changed the height of the item. However, try this. Figure out the height of the submenu off which the next level opens, let's say it's 100px, in the second level submenu put top="offset=-100px"; that should open it 100px up from where it would normally open. There may be better solutions.
There is this
pageabout methods and functions which might help if you know how to do any of that. I know it has things about getting and setting positions.
Ruth