Bags of frozen chopped spinach dehydrate very well in the dehydrator--just spread the spinach on parchment-paper-covered trays. It's a good idea to break up clumps when it's halfway dried. Choppped dried spinach reconstitutes quite quickly so should be just fine using cold water (with a longer soaking time).

I've gotten to the point that I add a tablespoon of dried spinach to nearly all my dehydrated meals to boost the veggie content.

Do note that dehydration destroys vitamin C. For longer trips, a Vitamin C supplement (unless, like me, you take a multi-vitamin/mineral supplement daily anyway) is an excellent idea.