Have you tried to attach it to some other JS event? The documentation  says the following:
The question is if "onmouseup" is included in the etc. :)
Oh - I shoulda tried that!
<a4j:support event="onchange" rerender="foo" />
works *perfectly* - thanks!
As it happens, I've just tossed the elements into an <a4j:form> so I don't think I'll need <a4j:support>, but good to know how it works.
This rich faces stuff is very cool - it's *starting* to make sense - I'm a back end guy, so my GUI experience is about nil; I'm finding this JSF/RF stuff fun - really nice.
Off to try to figure out the event model in JSF.