http://meta.wikimedia.org/wiki/Help:Images_and_other_uploaded_files#Navigati...
describes a couple of ways to get the effect of image maps in MediaWiki. However, neither of these is going to be easy to use with images created by Graphviz, etc.
Are there any tricks that this page doesn't mention? Is there anything else in the pipeline? Inquiring minds need to know...
-r