Though I'm no longer sure it's worthwhile. The current non-validating solution causes no problem for anyone, 'cept with validation.
Point being; I'm not convinced that validation for it's own sake is valuable.
Creating a solution is relatively straight forward (data-* attributes in html5 is valid), but I would still have to support the old method for eternity. Plenty of users have hard coded rel="lightbox" in their content.
I'm currently working on the codebase for an upcoming refresh and I've got this thing rolling around in my head. Any input is welcome.