Use a fully qualified domain name; the UBB will screw up links trying to do what you're doing, IE:
<script src="http://www.yourdomain.tld/ubbthreads/jwplayer/jwplayer.js" ></script>
And I would recommend against hardcoding new UBBCode, the reason I convinced Rick to adding the custom tag editor was to cut down on having to do this and make an extremely simple method of adding new items (there are huge lists of custom bbcode before and after the tag editor at UBBDev).
