what the heck? DO NOT edit anything directly in the header-extensions.php file or ANY core thematic file. that defeats the whole purpose of child theming... which is pretty brilliant if you ask me. you didn't but i'm saying it anyway.
i JUST wrote how to use the override function so i will mostly copy from that post
take a look in the library/extensions/header-extensions.php file of thematic. my first step when i want to modify something is to figure out where it is coming from. 90% of the time it is coming from the extensions folder.... probably 100% of the time if the info is dynamic at all (like in this case).
you will find the thematic_blogtitle() function is the one that spits out the blog title and its link. so to change how to spits things out you'll want to create an override function in your child's functions.php
function childtheme_override_blogtitle(){
//stuff
}
i usually copy over the original function into my override and then hack away from there. the blog-title link comes from the home URL of the wordpress install. the link you want to use seems outside of wordpress? the easiest thing would just be to hard code it into the override
it will be same process for 80% of the things you want to change w/ thematic...