I can't find documentation anywhere for formatting an array properly in
the launch file.

This does not work
<param name="marker_center" value="[0.0, 0.0]"/>

This works
<rosparam command="load" file="marker.yaml"/>

where marker.yaml is

marker_center: [0.0,0.0]

Any ideas as to what I'm doing wrong?

Thanks,
-Bill