I would remove the boards and find another use for them before just covering them up.What do you have against drywall?All you want to do is just cover it with wallpaper anyway.
Thanks for pointing out the obvious,I didn't realize That I was repostng the same thing .
In a balloon type building I would not suggest removing the shiplap planking as it is part of the structural integrity of the building. I have the same problem and adding drywall is a pain. To do it proporly you have to remove all the wood trim (baseboards and window) and put spacers in so that the trim still stands out after you have put down the drywall. This is major work.
I had considered drywall mud to smooth out the walls but was told that with the expansion and contraction from humidity and the swaying in the wind(it gets to 80 plus mph here) that wouldn't work either as would be the case with actual plaster. Soooo I am going to use, it is called, uh oh I forget, but there is a product I think called sizing paper that is like sewing interfacing that you use on rough walls prior to wallpapering. It comes in 36" wide rolls and I hope some wider. You use wallpaper paste and put it down first then the wallpapaer. This replaces the muslin.