I looked at this. The problem is that non-tail try blocks turn into closures, and thus the field gets propagated as a constant. IOW this can't work:

(deftype Foo4 [^:unsynchronized-mutable x]
  MutableX
  (set-x [this v]
    ((fn [] (set! x v)))
    v))

I'm not going to re-evaluate the try/closure approach right now, so I recommend you make a helper method that just does the assignment and then call that in the bigger context, as a workaround.

Closing this as it requires more than a simple bug fix. If you feel that Rich's work-around is unsatisfactory please create a clojure-dev discussion about rewriting try-catch.