\version "2.13.47"

#(define-public (symbol-or-markup? x)
  (or (symbol? x) (markup? x)))

#(define-markup-command (mytest layout props label arg)
  (symbol-or-markup? markup?)
  (let* ((s (if (symbol? label) (symbol->string label) (label))))
    (interpret-markup layout props  (markup #:concat ("Label: " s " arg: " arg)))))

% works:
\markup \mytest #'test \bold "f"

% any markup or markup function does not work:
\markup \mytest "test" \bold "f"
\markup \mytest \bold "test" \bold "f"
\markup \mytest \markup { "test" } \bold "f"