A few lines of elisp to instruct slime to ask swank to use the asdf fasl cache directories when it compiles things. For example using C-c C-k.

slime-plz-use-asdf-cache.el

;;; elisp that instructs slime/swank to use the ASDF cache for fasl files.
;;; add this to your emacs init file; and the magic happens.
(defun setup-to-save-swank-fasls-in-asdf-cache () 
  ;; based on http://article.gmane.org/gmane.lisp.slime.devel/11080
  (slime-eval-print "
     (flet ((find-fasl (path options)
                       (declare (ignore options))
                       (let ((fasl-path
                               (asdf:apply-output-translations
                                   (compile-file-pathname path))))
                         (when fasl-path
                           (ensure-directories-exist fasl-path)
                           fasl-path))))
       (let (fasl-finder-sym)
         (when (and (find-package :swank)
                    (setq fasl-finder-sym
                          (find-symbol \"*FASL-PATHNAME-FUNCTION*\" :swank))
                    (null (symbol-value fasl-finder-sym)))
           (set fasl-finder-sym #'find-fasl))))"))

(add-to-list 'slime-connected-hook 'setup-to-save-swank-fasls-in-asdf-cache)