This type of change would typically be done with a plugin. Keep in mind that this isn’t a BuddyPress customization as much as it is a WordPress MU one.
I did a quick Google search for “change wordpress admin profile link” and found a few places you could start.
In this WordPress forum topic is some code that would change the behavior of the profile link (albeit for a different reason); this method of changing the admin behavior isn’t recommended, however, since it requires hacking a core file. A plugin would be much better since it doesn’t require changing the core file (and having to stress about updating your WordPress MU code later).
Depending on how much you want to work on this, you could download these two plugins: WP Hide Dashboard and Admin Menu Management, open them up in a text editor, and look at how they change the menus in the WordPress admin area.
Plugins do two things: (1) they provide a new functionality and (2) they hijack the normal WordPress behavior and perform the functionality described in the plugin instead.