What is an aesthetically pleasing way to add lateral support to a loft bed?
Since you are doing sturdy x-bracing under the bed itself, and two adjacent sides are cross braced as well, you have three braced frames in three planes, and none of the frames are overly long and skinny, there is adequate bracing for the entire structure. If you can in any way anchor the structure to the walls as well for additional rigidity, you will be better off.

Even if you can't, as long as the frames are truly rigid and do not rack in the least, you will be fine. The one unbraced post is a bit of a problem unless anchored at the base because it can move if bumped against. If bumped hard enough, you could lose the vertical support at that corner. Thus the connection of this post must be as rigid as possible at the top connection to the bed frame. It's essentially like a table leg, a loose one is nearly useless.