You can use data parameter as function. Simple example of stripping HTML out:

$(".editable").editable("http://www.example.com/save.php", {
    type : "textarea",
    submit : "OK",
    data: function(value, settings) {
        return  $(value).text();
    }
});

Your sugestion didn't worked here, i used this instead:

data : function(value, settings) {return $('<div />').html(value).text() },

But even this doesn't work properly on IE on TextArea, where when source text has Line Breaks
, as it strips all lines into one...
In IE it's better to use .html() not .val() on TextArea
So i've created a new type for textarea where:

content : function(string, settings, original) {
  $(':textarea:first', this).html(string);

},

And used this on TextArea new type:

data : function(value, settings) {
  return value
    .replace(/<br \/>\n/ig,'\n')
    .replace(/<br>\n/ig,'\n')
    .replace(/<br \/>/ig,'\n')
    .replace(/<br>/ig,'\n')
},

Hope this help...
Thanks