Is this new? I was checking the status of my sitemaps in webmaster tools and found a page that says I can submit one sitemap, from anywhere, that cover links to pages on all of my sites if those sites are verified inside GWT.
it is usually best to keep things as simple as possible to have maximum search engine crawling. the bots are getting smarter every day but there are still many, many issues that can cause your website not to be indexed.
its not always possible to avoid the more fancy techniques but simple does equal less headaches.