I have asked for that as well. My workaround for designing is to keep a late game save around for designing purposes. Just remember to save after designing and you can design away. The ships are there for your next game. It is still not easy to arrange a fully fleeted battle viewer battle. And I can't count on the AI to cooperate, of course. They are always doing something disagreeable. It's like they were coded that way, or maybe they're disgruntled teenagers.
Considering that a battle viewer would be a big thing, I would have expected them to be touting it loudly along with other features, which makes me feel that we won't be seeing a battle arena with Crusades. If I am wrong, then they are keeping it for a surprise, and probably won't say anything about it until release. It's a thin hope, but it's the best I can offer.