New nivo slider has an HTML captions option, where instead of putting the title into the title attribute, you put an html ID into it, that points to a div containing html for your caption.
I think that adding a caption field to the view attachments_after could accomplish this with a fiew minor tweaks to the row preprocessor.
I set up a module that takes a 4th field option as a caption, and puts it in a
that contains the title.
It would be cool to see it tested and possibly added to the next release (if it's not already being worked on). I'll continue testing and post back here any issues.
(see this possible problem: http://drupal.org/node/1125018)