We could add BOARD_LOCAL_URL and prepend it with the data you suggested and advice using that when suitable and keep LOCAL_URL as is.
Although we could also use LOCAL_URL and break all existing BBCodes:
A local URL. The URL must be relative to the topic page and cannot contain a server name or protocol.
An Admin can then change it to the new RELATIVE_URL, when ever it's needed.
Advance here is that stuff is broken for security reason and can be fixed within 1 minute (could also be notified while updating the board to check it). Rather then leaving everything and hope that the admin will check the BBCode acp anytime soon...