Good point.  Two modules on the same page would be a problem if they had nodes of the same name (otherwise the pages could all exist on the same level) .  While rare and probably poor information architecture I can see it creeping up. Here is some thinking to start if off...

Example: a blog, and article app are on the same page called "Resources".  Both have an entry called "My Posting".  Currently the breadcrumb would stay:
Home > Resources
regardless of the visitor's actions. 

A simple extension of the breadcrumb won't work:
Home > Resources > My Posting
as it doesn't indicate the module.  This was an issue for friendly URLs too.

What if the title of the module was the base of their branch? Then you could extend the breadcrumb as such:
Home > Resources > My Blog > My posting, or
Home > Resources > Articles > My posting

This works for URLs but usability is different on page.  "Resources", "My Blog" and "Articles" are now redundant links to same page but will different apparent meaning to the visitor.  That replaces the problem with a different poor experience.

What if the tab and module were combined into one link?
Home > Resources: My Blog > My posting, or
Home > Resources: Articles > My posting

What if that was also the format for variables?
Home > Resources: Articles > Category: Personal > My posting
(The colon character would be a default but configurable by the skin designer)
(Probably best if category and categories were seperate links as they are different pages. Ultimately, what is used in the breadcrumb would be up to the module developer)

Or, is it fair, correct, desirable or possible to limit the number of site map extending modules on a page to one?  What if the administrator was particularly poor and named the modules identically?