Coding script EventHandler for a 'x' close button of a floorplan

Hi,

I am trying to extend a lower layer UI QA floorplan to add functionality to the 'x' close button present in top right corner of the page.

Requirement basically is to open a modalDialog upon clicking the 'x' button (based on condition that will go in the eventhandler script).

When I create the xbo for the lower layer BO and click open the right screen using Enhance Screen, in the Extensibility Explorer, I don't see an anchor that may help me modify an event handler...

My question is...

a) are we allowed to modify/add scripting to lower layer buttons?

b) If yes, how do we find out whether a particular floorplan allows extension on that button.

c) If no, is there a different approach to design that I should be taking?

Thanks,

Vineet.