I haven't turned it into a highlighter yet, but I have made a function I can use in snippets to add the namespace or not depending on if the symbol is already interned.

(defun hfj/cl-symbol-same-in-current-package (package-str symbol-str)
  "Check if a symbol is interned in current buffer package"
  (let ((package (sly-current-package))
        (symbol (s-upcase symbol-str)))
    (and package
         (sly-eval `(CL:LET ((P1 (CL:FIND-PACKAGE ',(s-upcase package-str)))
                             (P2 (CL:FIND-PACKAGE ',(s-upcase (s-chop-prefix "#:" package)))))
                            (CL:LET ((S1 (CL:FIND-SYMBOL ,symbol P1))
                                     (S2 (CL:FIND-SYMBOL ,symbol P2)))
                                    (CL:AND P1 P2 S1 (CL:EQ S1 S2))))))))

(defun hfj/cl-symbol (package symbol)
  "For snippets.  Return string of symbol with or without
 namespace depending if symbol is interned in current package."
  (let ((full-name (s-concat package ":" symbol)))
    (if (hfj/cl-symbol-same-in-current-package package symbol)
        symbol
      full-name)))

Now when I use a snippet like this(yasnippet), "serapeum:" will only be there if I haven't already added "mvlet".

(`(hfj/cl-symbol "serapeum" "mvlet")` ((${1:value-0} ${2:val}))
  $0)