Yep, basically since buddypress is a plugin it should work as a plugin and other blogs should have the choice to use it or not, or as the webmaster, you should be able to choose which blogs have it and which blogs don’t. Seems like a simple idea. I’ve spent weeks on this as well.
But your approach seems to be in the right direction. $blogusers = get_users_of_blog(); Are you using one template or multiple templates for each site?