Representing a queue as a procedure with local state
You did not provide implementations of the queue operations using this representation. In SICP this usually refers to syntactic sugar to make working with a data structure less awkward, e.g.
(define (queue-insert! q item)
    ((q 'insert!) item) )

Also I would prefer different operation names, e.g.
append! instead of insert!
front instead of front-ptr
...