Similar to the available XML feeds an automatically generated sitemap in XML format as specified at https://www.google.com/webmasters/sitemaps/docs/en/protocol.html
should be available. It should contain only publically visible pages.
If and how some fields should be filled is to be discussed:
priority could be created from the namespace depth. Eg. the toplevel namespace gets a 1, the next namespace gets a 0.9 and so on... But I'm not sure this is the best idea
changefreq could be either set to a default, the cachetime or maybe even calculated from previous changes.