First of all you MUST add this attribute to the form (doesn't matter where), but it should be hidden. To hide an attribute add the following to the script of the attribute:
markup.css ("display", "none");
Then to access it use this in the render script of the form:
var field = parser.getField ("AttributeName");
To get the current value:
var value = field.getValue ();
I think all of this is documented in the Programmers Reference Guide, but I cannot be sure.
So:
if (parser.getField ("status").getValue () == "Complete")
$("#" + parser.m_widgetInfo.wrapperId).find (".aw-form-buttons .k-primary.k-button").css ("display", "none");