here's an easy way to add underlining

lines 989-993 in signwriter.module changed to

else {
imagettftext($im, $fontsize, $angle, $x + abs($box[0]), $y + abs($box[5]), $foreground, $fontfile, $line);

if ($underline) imagettftext($im, $fontsize, $angle, $x + abs($box[0]), $y + abs($box[5]), $foreground, $fontfile, '______________________________');
}

It just adds the '_' across the width of the image (overtop the original text), which makes a pretty good underline!