So in this case I have ended up simply relying on javascript - using jquery...

noHtml($("textarea"));

// no HTML binding
function noHtml(el) {
    if(el.length) {
        el.bind("keyup blur", function(){
            var cleanText=stripHtml($(this).val());
            if($(this).val()!=cleanText) {
                $(this).val(cleanText);
                shakeIt($(this));
            }
        });
    }
}

// strip any HTML
function stripHtml(html){
   var tmp = document.createElement("div");
   tmp.innerHTML = html;
   return tmp.textContent||tmp.innerText;
}

// user feedback
function shakeIt($el) {
    if($el.length) {
        $el.animate({opacity:.2},100, function(){
            $(this).animate({opacity:1},200);
        });
    }
}

May not be the perfect solution but easy and effective at least... modify any to suit your needs- ie: i didn't want actually shaking in this instance.

Hope it can be of use to someone else.

Cheers
Josh