Not really an answer to your question, but more a hope that someone has built something that would be. The nicest tool I've ever used for creating GUIs that were mostly graphics with some clickable elements was JF Bartlett's ezd. See http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-91-6.pdf for more info.
The diagrams package is easy to use, but more powerful than simple. With the gtk backend you can get points from mouse clicks, The diagrams user manual points out that the query feature of diagrams could use such clicks to identify a set of elements that were clicked on, which could then be used to find the code you want to run. So in theory, you could use diagrams in a manner similar to the way ezd worked - but I couldn't seem to find an example of this use.