One old technique that might work here is the “let in ribbon” or “cut in ribband”. This traditional method was used in “balloon framing”. Before the wide adaptation of “platform framing” with the advent of air conditioning - there was no top plate to rest your joist on. This still meets code if done properly:
R502.6 Bearing. The ends of each joist beam or girder shall have not less than 1.5" of bearing on wood or metal and not less than 3" on masonry or concrete except where supported on a 1x4 ribbon strip and nailed to the adjacent stud or by the use of approved joist hangers.
In balloon framing walls were hollow and allowed the hot air within to rise to the attic. Acting as a chimney, along with plaster walls retaining the cooler evening temperatures and typically much larger windows, balloon frames provided passive cooling but were also fire hazards.
A stub mortise is cut on interior facing edge of the studs just below the bottom of the joist. The joist passed over this ribbon and was nailed to the stud. The ribbon transferred the floor load to the stud. The ribbon itself was 5/8” x 10” to 1-1/2” x 6” nominally. Some retrofits used steel strips which were thin enough and secured with screws so as to avoid having to mortise the studs in place.
In your project, you could snap a chalk line around the wall, cut through the plaster and add the depth of the ribbon then chisel away the stud cavity, set the ribbon in and be left with an interesting molding opportunity. I have used steel stringer channel which is relatively lightweight and provides a flange for the joist to bear on. The channel can be painted and left exposed obviating the need for major plaster work.
