As far as I can see image and iframe are the only way of achieving this, though you could possibly use dynamic css too; either way, your signature generation API would need to change, or at least be wrapped.
A blog post here details how the author went about doing just this for posting dynamic scores on a user's Facebook wall.
This is the workflow they used:
As the score had to be dynamic, what they did was create an API to take a snapshot image of the HTML content which the client could then show.
They created an API endpoint which would
Query the score HTML generator endpoint (in your case signature generation) to generate the HTML you want the client to display.
Turn the HTML into PDF using the HTML2PDF utility for PHP 5.2.
Render the PDF into an image which can then be returned to the user for displaying using PHP::ImageMagick.
They did this with the following code:
$html2pdf = new HTML2PDF('P', 'A4');
$html2pdf->writeHTML($html_content);
$file = $html2pdf->Output('temp.pdf','F');
$im = new imagick('temp.pdf');
$im->setImageFormat( "jpg" );
$img_name = time().'.jpg';
$im->setSize(800,600);
$im->writeImage($img_name);
$im->clear();
$im->destroy();
The image returned by this API can then be rendered into an image tag for display to the client. There is a demo of this workflow in action available here.