I am trying to make a Zimlet that will match a re for a decimal value, and on a hover load a window with the contents of http://foo/bar?[the value matched for the regular expression]

At present I have just com_example_foo.xml in the zip; its contents are:

Code:
<zimlet name="com_example_foo" version="1.0" description="Information Lookup">
    <contentObject>
        <matchOn>
            <regex attrs="ig">[regexp here]</regex>
        </matchOn>
        <toolTip sticky="true" height="300" width="300">
            <actionUrl
              target="http://foo/bar">
                <param name="query">${src.objectContent}</param>
            </actionUrl>
        </toolTip>
    </contentObject>
</zimlet>
At present the Zimlet seems to be loading and the regular expression seems to be matching what I want it to match; a test identifier in the document turns blue, and a toolTip pops up, but it says "undefined." If I manually test the URL it should be pulling from, it gives something other than "undefined," whether or not any parameters are passed.

Am I missing something about the right way to make a mouseover window? I couldn't find a particularly complete <toolTip> example in zimlets-extra or zimlets-experimental. If I can't or shouldn't use <toolTip> to do this, what are my best odds for making output from http://foo/bar easily available in connection with an appropriate identifier?

Thanks for any pointers,