I have successfully installed a buddypress/wordpressMU install in a sub directory as a test.
When I have tried to replicate this on the URL I want to do it on, I just can't.
How difficult would it be to move the test install to my live site?
(they are on the same server, just different URLS)
Maybe I could just park my URL on the sub directory?
It's relatively easy, although you'll need to change a whole bunch of stuff in the database plus the blog's location in wp-config.php (which, by the way, you shouldn't touch under normal circumstances, unlike 'normal' Wordpress).
I did this last week - moved an installation from one domain to another, although moving to a new directory shouldn't really be any different as far as I know. It took a bit to get all the appropriate bits changed but the overall process was easy.
Try http://www.kristarella.com/2009/05/how-to-move-wordpressmu/ - it's recommended by andrea_r, the WPMU guru. :)