You can use inline CSS on the html element that displays your message.
For example, here is something I do. As long as the token $no_data_chart_msg$ is defined, it will appear in place of the chart, and it is moved down using the margin setting.
<panel>
<title>...</title>
<html depends="$no_data_chart_msg$">
<h3 style="margin: 100px 0 0 10px !important;">$no_data_chart_msg$</h3>
</html>
<chart rejects="$no_data_chart_msg$">
....
</chart>
</panel>
If you want to center the message in the horizontal middle of the panel, you will probably have to get creative with the CSS using auto on the margin-left and margin-right settings.
