Lightbox without rel tag or addRel without jQuery

Question

This is the idea. I want to put a lightbox (or slimbox or mediabox or whateverbox) on my forum so that it will open user-posted links in lightbox.

Problem is, lightbox requires the rel=lightbox tag, and people are too lazy to use the custom bbcode that adds the correct rel tag to the link.

So, can lightbox (or any other) work for all links without the rel tag?

The alternative is to add the rel tag to all links. I can use the 'addRel' function from jquery, but it would be a waste to include the whole of jquery only for this one function.

How can I add a rel tag without jquery?