Never mind, I take back my proposal - when I think about it some more callbacks wouldn't have helped me anyway.
Actually, this is solved by the new check I'm using in site-master.tcl, which finds out if any curriculum instance is mounted in the sitemap under the current subsite, in order to know if the bar should be displayed or not.
See this thread:
https://openacs.org/forums/message-view?message_id=104815
