Centering a sub-popup menu on an item

[Mike Taht <m taht net>]

I am thinking in my carpal fashion that centering sub-popup menus on an item 
would be a good thing, if doable in a gtk app. 

Half the wrist movement to go up or down. Better use of screen display area
etc. (and see tog's question's 6 and 8 at: 
http://www.asktog.com/columns/022DesignedToGiveFitts.html
for more justifications)

But forget about the justifications for a bit, I'm trying to figure out
how to do it. If I were to write a 

void                (*GtkMenuPositionFunc)              (GtkMenu *menu,
	gint *x,
	gint *y,
	gboolean *push_in,
	gpointer user_data);

and pass in, say, the name of the menu item to center around in user_data,
 how would I go about getting the effective x and y offset from the top 
of the menu of the menu item before its realized?