The content option searches for stuff inside packets. So if you need to mark routing just for single packets (e.g. udp dns queries), it can work. But not for anything more complex, e.g. whole tcp is out, because the content will be only in one packet and not in the rest belonging to connection. Even if you’d want to mark connection first, it still wouldn’t work, because content would not be in very first packet, but only in some later one. And once the connection goes one way, you can’t change it.
Indeed, I tried with connection mark, without success. It seems that only way to do it, is write a script which will ask dns for ip addresses for this domain and mark routes according to address list.