Big picture of how I would do that, but maybe it can be a better way:

• Create a filter into the FIRST_FILTER section of the web.xml (contribute into the deployment-fragment.xml, target=“pages#PAGES”)
• Into this filter check if the user is not connected (must found in session the principal here ((CachableUserIdentificationInfo) req.getSession().getAttribute(“org.nuxeo.ecm.login.identity”)).getPrincipal() ) let the filter chain computation following his computation
• if the resource is open (image, gif) => open urls are given here :(PluggableAuthenticationService) Framework.getRuntime().getComponent(PluggableAuthenticationService.NAME).getOpenUrls() and check if match: openUrl.allowByPassAuth(request) => if match let the filter chain computation following his computation
• get the directory where you store information filled by users (Framework.getService(DirectoryService.class).open(“nameOfMyDirectory”) => return a session on the directory)
• search from the given session the entry of the user. If not found let the filter chain continue his computation
• if not redirect to your xhtml page that expose the form.

Hope this will help you. Maybe there will be a better way but to be sure that no user skip this page, I don't see no other way.