Like with-redefs but local to the thread. Doesn't require dynamic function vars.

with-redefs.clj

(def ^:dynamic *hook* false)

(defn hook [var f]
  (alter-var-root var
    (fn [original]
      (fn [& args]
        (if *hook*
          (apply f args)
          (apply original args))))))

(defmacro with-redefs*
  "Like with-redefs but scoped to only the
  thread of execution."
  [bindings & body]
  `(let [m# ~(zipmap (take-nth 2 bindings)
               (take-nth 2 (next bindings)))]
     (doseq [[k# v#] m#] (hook k# v#))
     (binding [*hook* true] ~@body)))