Apache ErrorDocument with absolute path
I don't think this can be done directly inside the statement: The ErrorDocument will always have to be relative to the DocumentRoot. According to the docs, the only alternative seems to be specifying an external URL, but that is bad because the wrong response header gets sent (302 instead of 404).
You could try whether anything is possible using an Alias directive or - I'm sure this would work - a symbolic link:
ErrorDocument 404 /symlinked_page.php