Perhaps I\'m going about this all wrong (and please tell me if I am), but I\'m hitting my head against a wall with something that seems like a really simple concept.
You can derive from HttpTextWriter and override OnTagRender, OnAttributeRender and OnStyleRender methods and modify the tags as they are rendered. The MSDN docs for OnTagRender show a sample where the tag is modified during rendering:
http://msdn.microsoft.com/en-us/library/system.web.ui.htmltextwriter.ontagrender.aspx