Security Trimming via SiteMap using authentication status?

Question

Does anyone know if there is a way to use a site map to perform security trimming based on authentication status?

For example, we can trim based on role using

<SiteMapNode title="Page" roles="Administrator">

Or allow anyone with

<SiteMapNode title="Page" roles="*">

But I have a series of pages for logging in, registering, etc. that I want hidden once logged in. So I need something like one of these:

<SiteMapNode title="Page" roles="(none)">
<SiteMapNode title="Page" authenticated="false">

Is there a way to do this?