Okay, I know this thought is really out there, but bear with me for a minute:
I think the wood driven into the corner studs is a way to mark off how deep the base coat plaster should be.
In those days, plaster was the one thing that could be used to true up a room or wall. Considering how much green, native lumber was used in those days shows how out of square a wall or room could be. Supporting this theory is that the OP may have had to shim out the wall with new 1 x stock to make them true.
Sadly, I've seen a number of people destroy perfect plaster walls that were as solid and as flat as you could ever imagine, only because it made them feel like they were "doing something with these old walls" while they "were fixin' this old place up". Once they had painstakingly destroyed the old plaster, they found walls that were so wavy that you could surf down them, and not a one of them true or square, especially for new drywall or trim. And then they cuss the old house because it was so out of square, and fill 1" gaps between the new trim and new drywall with a tube and a half of caulk.
I don't know if anyone else has worked much with new plaster for restoration work, but I've found it can be a little hard to figure out where to bring your scratch coat, especially if you are coating a wall of lath without any other reference point. It certainly takes a while to get a feel for the depth when applying your base coat.
It is my estimation that the wood driven into the corners, and not found anywhere else is to allow the plaster men to gauge two walls at the same time. Otherwise, one wall might be 3/4" thick. another might be 1/2" thick, and yet another, be 3/8" thick, all in the same room.
These wood shims might have also allowed the plaster crews to start a house with the base coat before the trim carpenters got to the job. It was standard procedure then to hang the trim first, and then start the plaster after the trim was finished.
Those wood shims might have been a reference point for lath crews, plaster crews, or the trim men, but my guess is that they were for the plaster guys for depth and keeping a wall true and square.