It would be nice if the URL bbcode could support relative links.
For sites that offer both SSL and non-SSL access to their forum, this would enable local links generated with the URL bbcode to stay in SSL or non-ssl mode for visitors.
Right now, it appears the URL bbcode doesn't support links that start with "/", which would indicate a relative or local page.
Another pie in the sky suggestion would be to strip out the matching "Board URL" from links generated by the URL bbcode, and make them all relative links, but that's probably a bit harder.
The only real solution to avoid this is to force rewrite all URL's to the SSL layer for everybody.
Let me know if any of that doesn't make sense.