Hi Marianne, I can give you an empirical conformation that paper clay can work when repairing bisque: A few days ago I dropped a small white stoneware clay sphere, belonging to a student, it shattered into more than a dozen small pieces, I was mortified but set to to try and rescue something from the debris. I used a stoneware flax 'paper clay' as 'glue' between the joins but when all of the parts that I could find were assembled a gaping hole still remained (approx 20mm diam); I managed to patch the hole with more paper clay but as DKat suggests overlaid this with some glaze. This morning I took this jigsaw sphere from the kiln and although some areas of the repair were visible, the finish was not too bad and the structure of the piece was sound.
Success in this case may be due to method: I coated the inside of the sphere with a moderately thin layer of the paper clay (up to the point where the inside was no longer accessible) and did likewise on the outside, I then brushed on a coat of glaze, before the paper clay had dried.
Andy