Not sure if this should go into http://code.pediapress.com/wiki/report too, but:
Allow cross-wiki article collections with the Collection extension.
People might want/need to have a mixture of Wikipedia, Wikisource, Wikibooks, Wiktionary and Wikiquote articles within one book! Having to make separat books for each project is not really nice.
So, somehow allow this; maybe with a temporary import-alike feature?