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You can use bp_parse_args() to do that.

So for the members loop, the filters are

bp_before_has_members_parse_args
bp_after_has_members_parse_args

I needed to only show members of a certain type in my directory, so this worked for me:

function only_members( $retval ) {
	$retval['member_type'] = array( 'type1', 'type2' );
	return $retval;
}
add_filter( 'bp_before_has_members_parse_args', 'only_members' );