So. I was on a wrong-side row on the afghan and tried knitting into each stitch to cast off. Didn’t love it. Undid it.
Thus the photos are of the finished right side (above) and wrong side (below) after purling-to-castoff from the wrong side at the top of the seed stitch edging. I like how it formed matching half loops over the purl bumps on the wrong side and how, on the right side, that straight line made a nice clean end there.
Note that if I’d been working from a right side row, knitting it would have gotten me the same as these.