"Hi @nomadic,

Yes, wpForo has filter hooks for changing the allowed HTML tags for posts.
Here is our filter for allowed HTML tags. You can add your custom function to our filter hook to manipulate the allowed HTML tags."

$allowed_html = apply_filters( 'wpforo_kses_allowed_html', $allowed_html );

Seems to be working, thank you! 
Here is a working example, in case someone else needs the same:

function custom_wpforo_kses_allowed_html($allowed_html) 
{    
    $allowed_html = array(
        'strong' => array(),
        'br' => array(),
        'b' => array(),
        'a' => array(
            'href' => array(),
            'title' => array(),
        ),
        'blockquote' => array(
            'class' => array(),
            'data-width' => array(),
            'data-userid' => array(), 
            'data-postid' => array(), 
        ),
        'p' => array(),
    );

    return $allowed_html;
}
add_filter('wpforo_kses_allowed_html', 'custom_wpforo_kses_allowed_html');