Customizing org-mode exports

Question

So, I have been using org-mode for taking my research notes for some time now. I love how I can seamlessly export to both latex (for my papers) and html (for my blog). However,

Answer 1

I usually have a file with many custom definitions which I reuse for many documents and I want to use it also in my org documents.

The following is a modification of Blout's answer, so please read his answer for more info.

Define macro

(defun org-dblock-write:insert-latex-macros (params)
  (let ((text)
    (file (plist-get params :file)))
    (with-temp-buffer
      (insert-file file)
      (setq text (split-string (buffer-string) "\n" t)))
    (insert (mapconcat (lambda (str) (concat "#+LATEX_HEADER: " str)) text "\n"))
    (insert "\n#+BEGIN_HTML\n\\(\n")
    (insert (mapconcat 'identity text "\n"))
    (insert "\n\\)\n#+END_HTML")))

Usage

File macros.tex:

\newcommand\a{a}
\def\b{b}
\DeclareMathOperator\c{c}

In org file:

#+BEGIN: insert-latex-macros :file "macros.tex"
#+BEGIN_HTML
\(
      \newcommand\a{a}
      \def\b{b}
      \DeclareMathOperator\c{c}
\)
#+END_HTML
#+LATEX_HEADER:       \newcommand\a{a}
#+LATEX_HEADER:       \def\b{b}
#+LATEX_HEADER:       \DeclareMathOperator\c{c}
#+LATEX_HEADER: 
#+END: