We have an old page written in Classic ASP that makes calls to an existing .asp page through the Prototype library's Updater object.
This call passed some query string parameters in w/ the request and received a list of params to update as well as a block of HTML/JS to render onto the main asp page.
It seems that the page lifecycle complicates things, but implementing in an actual server control or usercontrol could work.
[EDIT] I'd like to clarify/simplify the original question: Is there a way to render an ASP.NET control (.ascx or server control) in a non ASP.net page?
Ideally it would be called via AJAX and return a block of HTML Text - along with some way to transfer state to the receiving page.