Re: Useful BP and non-MU BP feature: list outside blogs
You could make a xprofile field that says “Outside Blogs” and they could list them there for now. They wouldn’t link to them yet however, and there’s an enhancement in the trac to allow for different formats of fields that will allow this later.
I suspect however that your goal is to have these blogs show in the “Blogs” area of BuddyPress. I think that’s a novel idea, and I don’t think it would be too difficult to do really, but it means that the blog area needs to be explored more, versus it only being a front end layer for all of WPMU’s blogs.
I think that they best they could be however would be links to outside blogs, and that keeping it within the content of your site would be difficult. (It would require an iFrame most likely, which I suspect most of us try to avoid.)