css classnames must be the usual identifiers (http://www.w3.org/TR/CSS21/syndata.html#characters)

In CSS 2.1, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [A-Za-z0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit.

Javascript doesn't mind, since you will use classnames as Strings. So you can use any character as far as javascript is concerned.

If you want to strip your classnames down to the usable css subset, a simple regexp should be enough. If you want to encode your classnames into the same subset, it will be a little tougher, but I suppose you can try to Base64-encode them. Here are some jQuery plugins for base64 encoding/decoding.

As far as the class attribute is concerned you will run into problems using chars other than [a-z], [A-Z] and [_-].

For arbitrary data I would recommend the (upcoming with HTML5) data-x attribute.
See http://ejohn.org/blog/html-5-data-attributes/ for an example.

Cheers