In a tableselect form element with '#js_select' => FALSE, how can I re-use the column header for a label? I have a pretty ordinary tableselect element that looks like so

$form['registration_table'] = array(
    '#title'=&gt;"Register",
    '#type' =&gt; 'tableselect',
    '#header' =&gt; $header,
    '#options' =&gt; $enrollment_options,
    '#js_select' =&gt; FALSE,
    '#multiple' =&gt; TRUE,
);


What I would like is to be able to use the empty space which is reserved for the "select all" checkbox for a text label. I would like to indicate by the label what action the checkboxes represent. In this case they represent selecting a course to register for so I would like to add the label "Register" there. Any advice on how to do this?
For doing this you have to change the "theme_tableselect" core code. See this documentation code part. If you do not set "select all" then the header is assigned to null