If I understand you correctly then a relay will do exactly what you want. I forget the exact nomenclature, but you want a relay with two sets of contacts one is normally open the other is normally closed. You would wire the 24V to the center contact and then each of the outer contacts will each go to the hot connection of your valve. When your X-10 is "off" the power will feed to the normally closed contact, when the X-10 is "on" the power will shift to the normally open contact. If that is not clearer than mud ask away and I will try to explain it better.