BuddyPress.org

I understand what you want to do, but there isn’t an easy way to do this. Members can name their blogs whatever they want, plus you can create blogs for whatever reason you want. BuddyPress is the only the exoskeleton of your site. WPMU is responsible for building the URL’s for your member blogs. The closest possible solution to what you want to do that I can think of, would be to insert “memberblogs/” before the content of each new blogs “site-url” in the wp_*_options table, but to be honest I’m not sure if it works that way.