It appears that the ability to change the rating of Gravatar used in BuddyPress is not exposed in any option form.
I managed to change it by editing bp-core/bp-core-avatar.php at line 158. Adding . ‘&r=<rating>’ a the end.
Some information here: http://en.gravatar.com/site/implement/url
If you wish, I guess you could add additional options to Site Admin->Options and insert the option into the database somehow.
If there is any plugin that does this for us, I’d very much like to know. Otherwise, I’m just sharing this tiny hack.
Cheers~