How I would go about it is based on the name on of the blog. If you go to the header.php file, there's definitely code that queries and then displays the name of the blog...that's how it knows what to display. Armed with that info, you can insert a few if-statements into the header.php file. I.e., if blog = "A", then echo "AAA", if blog = "B", then echo "BBB".
Should work. I'm working on something like this myself, but other issues are coming up. Namely that there seems to be a bug in the Every Home template. The post author is not passing to "the_author()", so the post author does not display over each post. Sucks.