Ken's imagemap idea could be built directly into the frontpage layout file rather than in section0 - this may give more freedom in terms of the code used as it could be built without worrying about the allowable tags in the editor or even using javascript etc.
Add the code to the centre section, just above the MAINCONTENT bit - you could even, for future ease of editing, create the imagemap code, links etc. in a separate tree.php file and just use a single include('tree.php') file in the main layout.
Lots of themes have used this kind of technique to add things like image sliders/marketing blocks and so forth, so I don't see any reason why the code for an imagemap and links couldn't be added in the same way.
Richard