It is similar to those 'master pages' you find in rapid website development. You can create a sort of template - including topmenu, header, etc. - and 'apply' it to the pages you want (not necessarily to all the website). Since C2 is strictly related to website creation now with html5, the Browser object ecc., it could be a great improvement.
I don't know if this can help. My solution would be to create gameplay layouts with an 'empty' upper layer (so you don't have to modify it later once per game level), then you create an event sheet (you call it 'UI sheet' for instance) where you create the GUI objects by actions and place them on the empty layer. You can include this event sheet in all the levels sheets, so you just develop once. Not sure about what may happen? Create an unaccessible layout where you play with the user interface objects on the screen so you can get an idea of what you will see (you can track down the object 'desired' positions and then create the GUI objects and place them at the same positions with sheet actions).
Sounds like a workaround, but at least you don't have to make the same change on a hundred levels <img src="smileys/smiley1.gif" border="0" align="middle" />