Another way to resolve my nightmare (alias pages: pages which "show" in a sibling blog) is to make a filter for the content, like "if page_title is listed in functions.php, take the content from another blog" (where the real page will be).
Have you ever seen something like this?
Do you have any leads to some similar filter?
Some details: I'd still use empty pages in the hollow blog because I don't know another way to tell wordpress to build menus, permalinks, etc, for a page in another blog. Also it'll help me to switch on-off the pages. So the content is in another blog of the same WP MS installation, under a same-name page, but it could be disabled just by un-publishing the dummy page.