I don't use the PHP version, but why don't you create each city board with it's own category, and this will give you jumper boxes on each board that contain the board for that city. Then create your Announcements board in an altogether separate category. And just create one Announcements board. Now go into the code where the jumper box is created with the select options for each board in that category and hard code a select line for the Announcements board. Now it will show up in the jumper box for every category/city.
Just my $0.02
