Just saw this reply now, this is not what I meant. (If it was, I guess I would avoid bothering you).
What I meant is that your plugin creates a page list, and than creates a sitemap. Naturally the 3rd party pages are added too, with the sm_buildmap action, however they are unable to access the page list.
Assume that my plugin is a one that creates a few pages from a single page. (you have example.org/page) and my plugin creates example.org/page/1 and example.org/page/2.
My plugin does this for every page, so a good use of the sitemap plugin would be to add such pages to the map. The suggested callback does not really allow me to do that, rather just add a few pages. When iterating over the list of pages your plugin generates (and adding such pages) would have been helpful.
Sorry for this long and complex message, I just hope I am understood this time. Thanks.