The attached patch adds a new functions org-export-as-string.

,----
| org-export-string is a Lisp function in `org-exp.el'.
| 
| (org-export-string STRING FMT &optional DIR)
| 
| Export STRING to FMT using existing export facilities.
| During export STRING is saved to a temporary file whose location
| could vary.  Optional argument DIR can be used to force the
| directory in which the temporary file is created during export
| which can be useful for resolving relative paths.  Dir defaults
| to the value of `temporary-file-directory'.
`----

This function should be useful in user code, and can already reduce the
amount of code in ob-org.el and org-mime.el.