Replying to myself again, as others might want to know. And BTW, I still want to know the correct way to do this; I fudged it.
I started with my circular needle at the armhole edge with the wrong side facing, and carefully picked up 46 stitches of the cast on edge with the left end of the needle, putting them on the right end. Then I slid the stitches along the needle so I could stat knitting at the neck edge, and knit those stitches (on the purl side). The result looks the same as the left shoulder, so I'm satisfied. But still... what's the "correct" way to do this?