Short of a drum sander, I wonder if a smoothing plane would actually be a better way. A small plane like a #3 should accommodate a bit of cup/bow without having to spend a bunch of time getting it flat first. Put some camber on the blade and set for a 2 or 3 thousandths cut and you can make pretty quick work of it. Actually, if you are going for rustic, then you can use a more heavily cambered blade and take deeper cuts, the texture left behind is more pleasantly rustic than coarse sanded.

Even if takes the same amount of time, it's much a much more pleasant way to spend shop time than with a respirator, earmuffs, shop vacs wailing, etc.

If you need to stain, it may not make as much sense because a planed finish won't take stain very evenly on pine, so you'll end up with the ROS anyways.