Security Trimming and SiteMap

[Guest]

I have a site that uses a combination of Sitemap and XMLSiteMapProvider with
RoleTrimming=true. It works great when secured for all roles defined for the
user.

Our site allows the user to run the site using only a single selected role
rather than all roles defined for him/her. I can obtain the selected role
but how can I tell the SiteMap provider to use only the privileges for the
selected role? I have researched this but it does not seem currently
possible.