I found an old, old thread about it, and it looks like the W3C considered it and decided that making a special case for 100% transparency was not the right thing to do. It's hard to argue, really. Should an image that is 99% (FE%? FE per FF? FE/FF?) transparent be clickable or not? It's not really any more visible. Perhaps it should only pass the click 99% of the time. Or only accept 1% of the click, merging the two links together in some mutant hybrid web page.

SVG has the ability to list parts of itself clickable and others not, but we both know the issues there. Too bad there's not a way to inherently tie an imagemap and an image together.