Linking to one of your sites could be a very nice way to pay for a blog, more tempting than cash to the blogger and (possibly) more valuable than cash to you. So why not add this feature to the "Pay to Blog" plugin where you could offer the possibility to link instead of paying with cash. Many people have several blogs in different blog networks and/or are members in different social networks like Twitter and Facebook. In the future perhaps different smaller networks (using BuddyPress) gain more ground. These all are exellent places for providing links to your sites.