It would be easier to manage exactly where header contributions are positioned in the head of the document if there was a placeholder tag which could be used to indicate the position. Much in the same way as wicket:child indicates where the child markup is positioned.
This idea from Martin-G.
See discussion at http://mail-archives.apache.org/mod_mbox/wicket-dev/201304.mbox/%3CCAMomwMqRds-PhWkR_%3DR1x-4HH9sSW41H1nARacU%2BN15uJHh9SA%40mail.gmail.com%3E
WICKET-5561 Derived page without own markup leads to MarkupNotFoundException when BasePage has <wicket:head> in its markup