#+name: plus-foo
#+header: :var buf-file=(buffer-file-name)
#+begin_src emacs-lisp
(concat buf-file "+foo")
#+end_src

#+RESULTS: plus-foo
: /tmp/example.org+foo

#+name: project-name
#+header: :exports none
#+header: :var root-dir=plus-foo(buf-file=(buffer-file-name))
#+begin_src emacs-lisp
(concat root-dir "+bar")
#+end_src

#+RESULTS: project-name
: /tmp/example.org+foo+bar