Found it. The Advocate and Family Guardian
"I suppose most of the readers of the Advocate and Guardian
have sewed, or are sewing for our army. The supply of hospital-stores constantly needs replenishing. Shirts and drawers, long and short dressing gowns and woolen socks are always in demand. Some the shirts should be made with only one sleeve and be tied or buttoned either on the shoulder or under the arm on the side without a sleeve, if buttoned on the shoulder they need not be open in front. Others should have large loose sleeves open the whole length, the shirts also open at the shoulders, and tied together with tape. The drawers should be made much like pantaloons, as they are worn instead of pantaloons in the hospitals. A few shirts and drawers of quite small sizes are needed for drummer boys, very few shirts larger than ordinary night shirts are required. The drawers should be as long as pantaloons. Old flannel is much needed as well as old linen and cotton."
That being said, this is the only
reference on Google books, so how common they might have been is anyone's guess. A normal shirt, after all, could be given to anyone, with one sleeve pinned as needed (on whichever side), while a one-sleeved shirt would only be useful to the one-armed soldier (with the side fastenings, you might get an ambidextrous shirt, so it at least wouldn't matter which arm was missing...).
You already mentioned having the USSC pattern, but if anyone else was looking for it (one of them?), there's flannel hospital shirts and drawers here