Automatic ping to search engines

[Carlos]

Instead of all that, maybe just use a global for the sitemap itself (xml or array or object), before creating it, call the hook, if after the hook the global is no longer empty then something already made a sitemap for us so we exit.

Yes, that's another possibility. A 'replace-sitemap' hook or filter.

I've sometimes thought about that, I intended to suggest someday a new type of hook/filter, not only for this sitemap thing, but for template tags like get_navigation, get_page_title, get_content, etc. Something like 'replace-navigation', 'replace-content', etc. GS would check if, after being executed, the corresponding global variable has some value, then stop if it has. This way plugins could customize those tags' outputs without needing to change anything in a typical default template (to make all this easier)