[development] Accommodating a second base_url

I'm working on a site that can be invoked via two different url's (mapped to the same directory). The requested url will dictate what top banner the site uses. Also, because there are likely many places in the site that $base_url is used, and the url appearing throughout the session should be that which was initially used, I will need to alter $base_url. So, I'm thinking I should just hook_init, and based on the http request, alter $base_url if needed, and do a variable_set that I later retrieve in the theme to alter (or not) the background-image used as the top banner. Is this the best approach? Jeff