Consider a #aero/resolve tag

[SevereOverfl0w]

This would utilize requiring-resolve (backported from 1.10 OR not available if not present). This would make it easier to support things like extending a protocol via metadata:

^{foo.bar/some-proto-method #aero/resolve my.project/proto-method-ext} {:foo/bar :config}

This would be assistive to projects like clip and edge which allow defining the system in Aero.

[nha]

I believe I have been using something similar in the past where configuration values could be functions (or indeed protocols), with the following reader:

(defn- require-resolve
  ([sym]
   (if-let [ns (namespace sym)]
     (when (ns-exists? ns)
       (let [ns (symbol ns)]
         (when-not (find-ns ns)
           (require ns)))
       (resolve sym))
     (resolve sym)))
  ([ns sym] (require-resolve (symbol ns sym))))

(defmethod aero/reader 'fn
  [opts tag value]
  (try
    @(require-resolve value)
    (catch Exception ex
      (println "Error resolving fn config value " value)
      (throw ex))))

However these days I tend to go for a small keyword->config map whenever possible:

(def ^:private m {:conf-value-a my.ns/func-a
:conf-value-b my.ns/func-b
:conf-value-c my.ns/func-c})

(defn- get-conf [kw] 
  ;; could throw here if there is no suitable value
  (get m kw))

A bit more verbose perhaps, but it works with other things like Java enums.