Here's the hook you want: 'PageRenderingHash'...
Thanks very much, Brion. Do you have an example of using this hook when you want multiple user options not to be cached? The examples on http://www.mediawiki.org/wiki/Category:PageRenderingHash_extensions are really minimal and involve cryptic things like "!" characters when the docs say to use a "hash", e.g.,
function ImageFilterHash( $hash ) ... $hash .= '!' . ( $wgUser->getOption( 'displayfiltered' ) ? '1' : '' );
Thanks, DanB