On second thought, I'd prefer the keywords to allow a single 
function or a list of functions.
This prevents breakage with current DOCT syntax and is cleaner 
IMO. e.g.

#+begin_srce emacs-lisp :lexical t
(let ((org-capture-templates
       '(("t" "test" plain (file "/tmp/test.org") "test %?"
          :hook (lambda () (insert "PASS"))))))
  (org-capture nil "t"))
#+end_src

See attached patch.