Feasibility of let bindings within defcustom

How can I apply (assoc-default 'GLXKS orellana-fpln) directly,
rather than relying upon galaxiakos-waypt?  Can one use a `let'
within a defcustom?

(defcustom galaxiakos-fpln
  `((GLXKS . ,galaxiakos-waypt)
    (OMNIX . ,(concat galaxiakos-waypt "/omniskon"))
    (PROTL . ,(concat galaxiakos-waypt "/protilus"))
    (GRGRS . ,(concat galaxiakos-waypt "/gregers")))
  "Association List (alist) specifying a flight plan, where each element is
a cons cell (WAYPT . DIRECTORY-PATH) linking a waypoint code to its
associated directory path for different Galaxiakos components."
  :type '(alist :key-type (symbol :tag "Code")
                :value-type (string :tag "Path"))
  :group 'galaxiakos)