See #3401 and http://orgmode.org/manual/Macro-replacement.html
% pandoc -f org -t native #+MACRO: HELLO /Hello, $1/ {{{HELLO(World)}}} ^D [Para [Emph [Str "Hello,",Space,Str "World"]]]
Inverted argument order
% pandoc -f org -t native #+MACRO: A $2,$1 {{{A(1,2)}}} ^D [Para [Str "2,1"]]