Okay, I don't have time to write up a proper update, but here's something....
This week Laev gave me quite a few proper sits during bitework, first in setting up for the blind search (which is classically hard for her) and then in heeling about and AWAY FROM the helper. In other words, exactly what we've been struggling with.
The possible solution? I went to "get her dressed" for her turn, but the previous dog took longer to finish than I anticipated and we had time to kill. So we did lots of sit-at-heel for clicks by the car, rewarding with food, with the constant lower stimulation of bitework on the field. She never failed to do it, but it was obviously harder for her in the beginning. By the time we got onto the field, however, something must have settled in her brain, because she heeled mostly nicely to set up for the blind search and then sat when cued. I clicked and handed her a last piece of food I'd kept back (surprising her, I think!) and then sent her around the blind.
After that, I split heeling away and turning to sit (as for the courage test) into finer bits, doing quarter turns and such, building up to the full 180 degrees. She isn't quite there yet, but it was loads better.
As someone else noted, "working on it on the edge of the excitement area, but not in it, and then repeating right away on the field, was the key."