Thinking about this a little more, using EXTRA_HEAD directly in a block isn't such a good idea since there may be lots of the same block on a page. I've fiddled with my code to use require_javascript instead, although I think this bug is still valid for those who do need access to the head.

Preprocessing now happens before the header's Tempcode generation. For most cases there was no issue, but EXTRA_HEAD was passed in as a parameter rather than referenced later as a symbol, so got bound early.