To take advantage of HTML5, we should really be using custom tags, not
attempting to embed data in a class (especially since gallery set, width,
and height data would need to be added as well!).  Moving to HTML tags is
certainly doable (such as <a data-mb_lightbox="true"
data-mb_gallery="set1"), but will require a bit of work on the internal
gallery system, which is beyond obtuse at this point.  Ideally, the entire
gallery system will be re-written to better support adding / removing /
managing media elements.  More along the lines of persistent arrays instead
of the current (and poorly defined) object system (a holdover from the
original Slimbox, which I can't even pretend to fully understand :D).  
Sadly, my free time never aligns with my ambitions, so this could take quite
a while to do.

The current plan is to keep backwards compatibility stable (maintaining
rel="..." usage), with an eye on HTML tags - if I can add them as an option,
great, if not, I'm keeping them in sight for future versions, no doubt about
it.  In the meantime, I'm going to have to recommend editing the javascript
yourself.  :(  It's not a good answer, but it's the best I can do for now
(especially since I'm not going to be implementing class="lightbox..."
support myself, it'd just be another misused attribute).

The two primary sections that will need to be modified is the scanPage
function (at the bottom of the script) which looks for elements with a
rel="lightbox..." attribute, and the anchor tag filter (about a quarter down
the page) called "linkMapper".  I haven't messed with those sections in a
while, so can't give you any more specific directions.  You should be able
to switch it over to class="..." if you really want to, but it'll require
parsing of the data to correctly implement the gallery, width, and height
settings.

Iaian7 / John Einselen