I now have my contrasting yarn for the Tomten Jacket borders. It is Elsebeth Lavold's "Silky Flamme'." (50% Wool, 30% Alpaca, 20% Silk). I think it is a very good match for my homespun Dragon Fruit. Here's a picture of the two together. I did a swatch with the black, and the size 9 (what I used for the pink) is a quite a bit larger (4.5" instead of 4"), and the 8 is just a tiny bit tight.
I made a test swatch, adding a border with mitered corners. It wasn't as wide as the actual borders, but it gave me an idea of what to do. I made the first attempt at the border using size 8 needles and a "lifted increase" (knitted into the stitch below the working row). However, it was too tight, and the corners too rounded. The "lifted increase" is supposed to be tighter and therefore more invisible, than other increases. On my second attempt, I knitted on 9's and purled on 8's. I also used EZ's "original" M1, per "The Opinionated Knitter," which is just a backward loop (like Sally Melville's "e-wrap" cast on) instead of a lifted increase or modern M1. (By "modern M1," I mean knitting into the yarn between the stitches on the previous row.) It is still a bit "rounded" on the corners, but much better than before, so this is what I plan to use.
I am going to start my pick-ups on the bottom at the "side seam" location (even though there isn't a side seam), and go all the way around the bottom, fronts, and collar. Every other row will be purl, because it will be garter stitch knitted in the round instead of flat. Mitered corner increases will be only on knit rows, starting with the third row. I'll need twelve ridges to get to size.