Thanks a lot, that pointed me in the right direction.
The following worked for me, please let me know if I'm making this harder than I have to :)
(defn is-type? [type]
(fn [exception] (= type (get-in (.getData exception) [:object :type]))))
(fact "create accepts custom validator"
(create :test {:key "value"} (validation-set (presence-of :foo))) => (throws slingshot.ExceptionInfo (is-type? :my.project/expected-type)))