I was just looking at the sendmail function of smf you can set the from parameter in the call but what it looks like it just sets the replyto not sure what it will do for bounces.
$headers = 'From: "' . $from_name . '" <' . (empty($modSettings['mail_from']) ? $webmaster_email : $modSettings['mail_from']) . '>' . $line_break;
$headers .= $from !== null ? 'Reply-To: <' . $from . '>' . $line_break : '';
$headers .= 'Return-Path: ' . (empty($modSettings['mail_from']) ? $webmaster_email : $modSettings['mail_from']) . $line_break;