This is not available in 1.4 or 2.0. We have added REST url support for several important dialogs etc. in Alfresco 2.1, including the task dialog. It could be done by extending the external access servlet, it is a little tricky as you need quite a lot of context information on the URL and you also need to setup a number of client beans correctly with the context before launching the dialog. The code to do this is available now in SVN HEAD, the class is called org.alfresco.web.app.servlet.command.ManageTaskDialogCommand.
Hope this helps,
Kevin