I don't see why cypress wouldn't be safe to use in an aquarium, I haven't really heard of certain woods that you're supposed to avoid. The real question might be how difficult is it to waterlog? Different woods take different amounts of time to waterlog in my experiences. I'm not sure if cypress being a tree that typically grows by rivers has any effect on it or not.
If it were me, I'd go ahead and buy some. If you can't get it to waterlog in a week or two, just attach it to a base.