How to create a customized logout URL in BuddyPress

That’s the same as the regular WordPress logout URL. So you can use the same filter: 'logout_url'.

Sample code, not tested:

add_filter( 'logout_url', 'wpse_56866_logout_url' );
function wpse_56866_logout_url() 
{
    // set your URL here
    return 'http://example.com';
}