What do you think of this approach based on
based on http://www.50ply.com/blog/2012/07/08/extending-closure-from-clojurescript/

While it may not preserve enough of gen-class's semantics. This would be enough for us to start gradually porting our large GClosure code base to Clojurescript. The code size reduction would be enormous.

(ns com.example
  (:require [goog.ui.tree.TreeControl :as TreeControl]))
(gen-class
  :name DemoTree
  :extends goog/ui.tree.TreeControl
  :constructor ([name config]
                 (goog/base (js* "this") name config))
  :methods [[handleKeyEvent [e]
              (goog/base (js* "this") "handleKeyEvent" e)
              ;; my special code to handle the key event
    ]])

here is a untested mock implementation modified from http://www.50ply.com/blog/2012/07/08/extending-closure-from-clojurescript/

I changed constructors to constructor because there can be only one in js

This unfortunately has different semantics from gen-class because the original did not include the definition of the methods and constructor inline. It tried to read the Clojure gen-class source,but I still do not yet understand how the :prefix grabbing of functions from the current namespace works from within a macro.

For Google Closure interop each class should have its own provide

(defmacro gen-class [{new-type :name base-type :extends ctor :constructor methods :methods}]
  `(do
     ;(goog/provide ~@(str *ns* "." new-type)) 
     (defn ~new-type ~@ctor)

     (goog/inherits ~type ~base-type)

     ~@(map
        (fn [method]
          `(set! (.. ~type -prototype ~(to-property (first method)))
                 (fn ~@(rest method))))
        methods)))