It turned out cutting through the knot was easy. I ran the jigsaw to within an inch or so of it, took the chance to reposition my board, and then ran it slowly through the knot. It threw out a bit of dark sawdust, much darker than the near-white dust from the rest of the board, and then carried on like the knot wasn't even there. Which was nice. And the side that faces out doesn't have any part of the knot showing, which is even nicer. So tomorrow I can dig out the rest of the window trim from the attic, and make a start on staining and lacquering. I'll need to do a bit of cleaning up first, I've got quite a lot of tools in that room that I've been using.
I think I can shift the oscillating tool back to the basement but need most of the rest of the tools a while longer. I'm really looking forward to getting my miter saw out of there and back onto its stand in the basement. I can't do that until I've cut the side pieces to length - where I've had to retrim the windows the trim across the top is dropping by about 3/4", and the new window sills are thicker than the old ones, so the side pieces will need to be shortened by probably about 1 1/4". I hope I can do that without dinging the finish. Maybe I'll run a craft knife over the visible side and use tape to limit the chip-out.