Hi,

Although I almost always use custom LaTeX classes and a separate file
for the preamble, I came up with this method to take advantage of
the 'latex' source blocks and write the entire preamble there. I guess
there will be a more elegant way to do it, but I think that it works
reasonably well ;-)

First, this function:

#+begin_src emacs-lisp
  (defun multiple-latex-header ()
      (save-excursion
	(goto-char (point-min))
	(while
	    (re-search-forward "_src\s+latex\s+:latexheader" nil t)
	  (when (equal (org-element-type (org-element-at-point)) 'src-block)
	    (save-restriction
	      (org-narrow-to-block)
	      (goto-char (point-min))
	      (let ((lines (split-string (replace-regexp-in-string "#\\+.+" "" (buffer-string)) "\n" nil)))
		(delete-region (point-min) (point-max))
		(insert (mapconcat (lambda
				     (line)
				     (unless (equal line "")
				       (format "#+LaTeX_Header: %s" line)))
				   lines "\n"))))))))
#+end_src

And based on this concept, we could also take advantage of the 'lua'
source blocks to generate the luacode environment (with or without
asterisk) and send it to the preamble:

#+begin_src emacs-lisp
  (defun env-luacode ()
      (save-excursion
	(goto-char (point-min))
	(while
	    (re-search-forward "_src\s+lua\s+:luacode" nil t)
	  (when (equal (org-element-type (org-element-at-point)) 'src-block)
	    (save-restriction
	      (org-narrow-to-block)
	      (goto-char (point-min))
	      (let ((luacode (save-excursion
			       (re-search-forward "\\(luacode\\**\\)" nil t)
			       (match-string 1))))
		(when luacode
		  (while (re-search-forward "\\(#\\+begin_src\s+lua.+\\)" nil t)
		    (replace-match (format "\\\\begin{%s}" luacode) t nil))
		  (while (re-search-forward "\\(#\\+end_src\\)" nil t)
		    (replace-match (format "\\\\end{%s}" luacode) t nil))
		  (let ((lines (split-string (buffer-string) "\n" nil)))
		    (delete-region (point-min) (point-max))
		    (insert (mapconcat (lambda
					 (line)
					 (unless (equal line "")
					   (format "#+LaTeX_Header: %s" line)))
				       lines "\n"))))))))))
#+end_src

And finally:

#+begin_src emacs-lisp
  (defun luacode-latexheader-filter (backend)
     (when  (eq backend 'latex)
       (env-luacode)
       (multiple-latex-header)))

   (add-hook 'org-export-before-processing-hook #'luacode-latexheader-filter)
#+end_src

It can be tested with this example that includes a simple function in
Lua (in a luacode* environment) to colorize the texts in 'otherlanguage', but
only in draft mode:

#+begin_src org
  ,#+LATEX_CLASS: article
  ,#+LATEX_CLASS_OPTIONS: [draft]
  ,#+LATEX_COMPILER: lualatex
  ,#+OPTIONS: toc:nil
  ,#+LaTeX_Header: \usepackage{luacode}

  ,#+begin_src lua :luacode*
    function foreignlanguage_draft ( text )
       text = string.gsub ( text, "(\\begin{otherlanguage}{[^%s]+})", "%1\\color{teal}")
       return text
    end
  ,#+end_src

  ,#+begin_src latex :latexheader
    \usepackage{fontspec}
    \setmainfont[Numbers=Lowercase]{Linux Libertine O}
    \usepackage[english,spanish]{babel}
    \usepackage{xcolor}
    \usepackage{ifdraft}
    \usepackage{lipsum}

    \newcommand\babeldraft{\directlua{luatexbase.add_to_callback
		( "process_input_buffer" , foreignlanguage_draft , "foreignlanguage_draft" )}}

    \ifdraft{%
    \AtBeginDocument{\babeldraft}
    }{}
  ,#+end_src

  @@latex:\lipsum[1]@@

  ,#+ATTR_LaTeX: :options {english}
  ,#+begin_otherlanguage
  Most GNU/Linux distributions provide GNU Emacs in their repositories, which is the
  recommended way to install Emacs unless you always want to use the latest release.
  ,#+end_otherlanguage
#+end_src

Regards,

Juan Manuel