As long as we're on the topic, I have to weigh in on the conservative side in this argument, with Ken Whistler. Use of the existing subscript characters is generally bad practice. Adding more subscripts would be adding to the bad practice, and yield even more different ways to express the same thing (markup versus direct encoding).
The examples given in the document are ample evidence that these linguistic notation systems have crossed way over the plain-text line. They should be *NOT* using the existing subscripts, but should consistently be using mark-up methods. Rick