RadControls for ASP.NET AJAX, as well as RadEditor, use custom rendering and applying custom attributes to be passed to the client is not recommended approach.
In general, when custom attribute is added to the control's attributes collection it is added to the HTML element representing the control on the page - the case with RadEditor, this custom attribute is applied to the controls wrapper <div> element and you can access it using the following approach:
editor = $find(
//get reference to the editor's client-side object
editorWrapper = editor.get_element();
//get editor's wrapper div
//access the custom attribute
For your convenience I have attached sample page demonstrating this approach.
In addition, please note that not every RadControl will honor custom attributes in this manner.
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