Last modified: 2014-09-07 16:59:02 UTC
Add hooks to make it possible for an extension to abort a page move. Probably there will be two hooks needed, one in SpecialMovepage.php's MovePageForm::doSubmit() and another in ApiMove::execute(). The move needs to be aborted before MovePageForm::doSubmit() deletes a target page to make way.
There is a AbortMove hook - https://www.mediawiki.org/wiki/Manual:Hooks/AbortMove
Ouch, that was some inadequate research on my part. Thanks for the heads up.