You would not find anything in the building code regarding blown in insulation in a cavity where knob and tube exists. But if you wanted to make a case where you wanted to point out it could not be done, that would be an electrical code issue. In this case since knob and tube wiring gives no temperature ratings, you could say it is not then suitable to be enclosed in thermal insulation since it is not marked as such. However I know there are hundreds of homes out there that would have insulated attics with knob and tube, not saying it is right.
