This is much trickier than I thought.
Apparently it is not working to use form tokens in the init section.
I got it working by putting the eval token section in the search of the drilldown dashboard:
<panel>
<table>
<title>...</title>
<search>
<query>...$form.Region$...</query>
<progress>
<eval token="form.Region">replace($form.Region$,"Total","*")</eval>
</progress>
</search>
Alternatively, which might be the better solution: Use a done section in the initial search to set a token which you use to populate the drilldown:
<panel>
<table>
<search>
<query>...</query>
<done>
<eval token="Region">replace($click.value$,"Total","*")</eval>
</done>
</search>
<option name="count">10</option>
<drilldown>
<link target="_blank">...form.Region=$Region$</link>
</drilldown>
... View more
