Yeah that's why I suggested 2 levels, and you basically outlined them.
1 level with important news that stickies on active topics and subforums, 1 level that doesn't sticky in active topics but does sticky in the relevant subforum.
Though having the second one may cause some who only use active topics to create another thread if they don't see the old one, but it probably wouldn't happen all that often and the benefit of less clutter might outweigh it.