Lua primitives: body()
When in message-mode, or index-mode, the body() function may be used to retrieve the parsed body from the currently selected message.
text = body();
If you wish you may point the function at a saved message:
text = body( os.getenv( "HOME" ) .. "/Maildir/Drafts/cur/1.2.3.txt" );
This function has been available since version 0.18.