r.a.d.spell is not limited to spellchecking html elements only.  You can define a custom text source, that can provide text for r.a.d.spell to spell check.

You need to define a JavaScript object with two methods: getText() and setText(param):

function VariableSpellSource()
{
    this.getText = function()
    {
        return "colourful mistakrn texr";
    }

    this.setText = function(newValue)
    {
        alert("The corrected text is: " + newValue);
    }
}

Going beyond that simple example, this page shows how to extract the text from a Macromedia Flash movie, containing a text area component, spell check it, and save the corrected text.  The custom text source is called TextInterface.

You can set it with the server side ClientTextSource property:

<radS:RadSpell ID="spell2" Runat="server" UseClassicDialogs="True" ClientTextSource="DifferentControlsSource" ControlToCheck="textarea1" IsClientID="true" ButtonLabel="Spellcheck and save to a different control" />

or the client side setTextSource method:

var source = new TextInterface();
spell.setTextSource(source);