The Ajax portlet typically uses a JSP or Servlet as a resource feed.
Problem with the reverse, is I don't know any way to invoke a portlet without going through the portal server, which triggers the entire theme/skin/etc to be generated. Plus, uniquely linking to a portlet might be a problem. Getting a URL to a specific portlet without getting anything but the portlet might be a problem - I'm not sure how you'd do that.
What I would say is that WSRP might solve this problem. It is a way to make a portlet available as though it were a web service. Linking in to the WSRP interface of a portlet (Web Services for Remote Portlets) might be the direction you're fishing in. WSRP is kewl, but seemed to have fallen out of favour for a while. It is a big part of JSR286, but we won't see any JSR-286 commercial portal servers for a while.