それっぽいものを投げやりに書いてみた。0-9a-zA-Zと-と_で64進。

(defun parse-integer/64 (s &key (radix 10))
(let ((table "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_"))
(car (reduce (lambda (c r)
(destructuring-bind (sum . pos) r
(let ((n (position c table :end radix)))
(cons (+ (* n (expt radix pos)) sum) (1+ pos)))))
s
:initial-value '(0 . 0)
:from-end t))))