test.clj

(when (not (find-ns 'mattermost))
    (.setDynamic                                     ;; <- This horrific interop into Var.java to counter
      (intern (create-ns 'mattermost)                ;;    CLJ-951, which is a closed/wishlist bug reporting that
              (with-meta '*me* {:dynamic true})      ;; <- this is actually not propagated fully (is, in fact, useless) :(
              "foo")))

(defn eval-w-bindings [str user]
  (binding [*ns* (create-ns 'mattermost)
            mattermost/*me* user]
          (clojure.core/refer 'clojure.core)
          (-> str read-string eval)))
        
(eval-w-bindings "(str \"hello \" *me*)" "andrew")

;; => "hello andrew"

This is what I wanted:

user=> (eval-w-bindings "(defn x [] *me*)" "someone")
#'mattermost/x
user=> (eval-w-bindings "(x)" "mjg")
"mjg"
user=> (eval-w-bindings "(x)" "jones")
"jones"

And the above code has does it! Yay! But I'd also like to be able to pass in the namespace instead of hard-coding mattermost everywhere.