BuddyPress.org

You can just switch out <?php echo site_url() ?> in you header.php and register.php theme files with you blog url.

~ of course it’ll break running the same default bp theme under multiple ‘sites’ (subdomains/directories). This may be a good enhancement ticket so that ~ if ( bp_core_is_multisite() ); $current_site or site_url can run under the blog url and not the site_url