authentication - Sitecore - Prevent access to a page, but still show it in the navigation

In Sitecore I have denied access to a particular page for the anonymous user.

This works correctly, but it also means that the page does not appear in the navigation menus and sitemap (both XSLT).

What I would like is for the user to be able to see the link, but be redirected to a Register/Login page when they click on it.

You'd need to allow anonymous users to view the page for it to be visible in the menu and sitemap.

What you could do though is override the Sitecore.Pipelines.HttpRequest.HttpRequestProcessor to check if the page requires a login (by adding a RequiresLogin True/False to the template for example), then check if the user is logged in if not redirect to your login page.

Theres an example of overriding the HttpRequestProcessor here

You could wrap the rendering logic in the menus in a SecurityDisabler, so it would render all item links, even if the user "cant see" them.

using (SecurityDisabler disabler = new SecurityDisabler())
{
  foreach (Item item in Sitecore.Context.Item.Children)
  {
    ... render the link ...
  }
}