I have done this in one of my outputs. Here is the code I used:
$structure['title'][] = array('text' => '',
'style' => 'rule',
'width' => '100%',
'y_relative' => 2.5,
'newline' => 'y');
This is what I had in my pdf.styles.inc file:
$style['rule']['font'] = array('family' => 'Times', // Courier, Helvetica, Times
'style' => '', // blank=Regular, B=Bold, I=Italic, U=Underline
'size' => 1.75, // size in points
'height' => 0.75, // line height in units
'draw' => 0); // width of drawn lines
$style['rule']['fillcolour'] = array(113,113,113); // colour for background
$style['rule']['textcolour'] = array(0,0,0); // colour for foreground
$style['rule']['drawcolour'] = array(0,0,0); // colour for line drawing
Easy peasy lemon squeezy (when you know how).