SpankMarvin on "Mobile Redirect with a difference."

It shouldn't matter where you place it in functions.php.

In the page editor, there should be a "Custom Fields" box. The name should be mobile_redirect and the value should be the URL.

Don't forget you'll need a function for actually checking if mobile. I had no idea there is actually a WordPress function for this, wp_is_mobile. So I'd modify the function to be:

function do_home_redirect() {

	if( !is_front_page() ){
		return;
	}
	if( wp_is_mobile() ){
		$redirect = get_post_meta( $post->ID, "mobile_redirect", true );
		if( $redirect ) {
			header('Location: ' . esc_url( $redirect ));
			exit;
		}
	}
}

add_action( 'init', 'do_home_redirect' );