As Tony says - the issue is with knowing where to place these custom class names, for exaample checkboxes and radio boxes would be quite fiddly with the label and input tags.
We may be able to do a bit of DOM manipulation to embed the custom class names into all the embedded 'input' type fields. So if you added a custom class (or classes) it would be pushed into all the embedded <input>, <textarea>, <select> tags but any other tags such as <label>, <span>, <button> would not be affected.
Would that be enough flexibility/functionality?