Wikiquote now has specific-language subdomains. Instead of creating wikis for all 150 languages, I made a system where wikis are only created when they're wanted. To save time during maintenance operations, it should now be possible to delete unused Wiktionaries and even Wikipedias until they are required.
Similar conversion of Wikibooks should be a fairly simple thing, I'm now just waiting for community approval. We can't put subdomains under wikisource.org because the domain name doesn't point to the right place. It's owned by Mav and points to a redirect server, redirecting to quote.wikipedia.org.
So, in terms of wikimarkup, you could use something like [[quote:Main Page]] to link to Wikiquote's main page from a Wikipedia or Wiktionary? What about [[books:Main Page]] or [[source:Main Page]]?