6
You didn't say which XOOPS version you're using, and that behavior varies with the version.
I doubt there's documentation on that, at least that's current.
In XOOPS 2.0.14, and possibly later 2.0.x versions, $xoopsOption['show_rblock'] is used in include/xoops13_header.php and footer.php. include/xoops13_header.php is conditionally included by header.php. So you might try setting $xoopsOption['show_rblock'] before including header.php, rather than after it.