BuddyPress HyperDB example

db-config.php

<?php
$wpdb->add_database( array(
	'host'     => 'global.db.example.com',
	'user'     => 'globaluser',
	'password' => 'globalpassword',
	'name'     => 'globaldb',
) );

$wpdb->add_database( array(
	'host'     => 'bp.db.example.com',
	'user'     => 'bpdbuser',
	'password' => 'bpdbpassword',
	'name'     => 'bpdb',
	'dataset'  => 'bp',
) );

$wpdb->add_callback( 'my_db_callback' );
// depends on your bp prefix bp_core_get_table_prefix() probably not working here
function my_db_callback( $query, $wpdb ) {
	if ( preg_match("/bp_/", $wpdb->table) ) {
		return 'bp';
	}
}