Adding a clickable area to the logo
When you add a logo this way, the entire image is a link back to the homepage.    The only answer I can think of is to place a link over the image.   using css the link will be


Float over the image using margin, position, etc
Text indent -9999px;
z-index higher than the logo img
pad the link to be the size of the image


I would probably custom code this in page.tpl.php, however it looks like you are looking for a non-custom code solution.   To do this create a new custom block, with the link being its content (remember to have a input filter that will take  HTML tags).   Move this block into header region.