And here's the proof. This is the center back piece with the horsehair back stay. I've cut the horsehair with half-width seam allowances and without the darts. The shoulderdart (which I think can look quite cool moved to the center back seam like this) has been cut open at its wide bit, pressed flat and hand-stitched (I don't know the name of this stitch in English, in Dutch it is called flanelsteek).
I wanted to get started last night, but I remembered that the only bit of horsehair in my stash was fusible. Yes, there is such a thing as fusible horsehair. I tried it out on a tiny scrap and it fuses really well and the end result gets quite stiff. Because I really want to try out the whole padstitching thing in this jacket, I didn't want to use it. So, I had to wait until today and go out and buy non-fusible horsehair.
One question for the experienced tailors among you: Normally, I cut the undercollar on the biais to let it ease into the shape of the upper collar. And when using woven fusible interfacing, I match that. Now, I want to shape that undercollar with horsehair and padstitching, so what do I do?
I know RTW jackets often have felt undercollars (which makes the whole grainline issue irrelevant anyway) but I have never taken one of those jackets apart so I don't know how those are shaped.
Oh, and by the way, do you really have to use silk thread for padstitching?
Unfortunately, I have a lot of other things to do today (it's my day off now, because I'll be working Saturday), like laundry, cleaning the house and some computer as well, so I don't think there will be any huge progress today.