I tried the Yarn Harlot's crochet cast off (thanks for the suggestion, tinebeest); this method works very well but the resulting edge was tight. I frogged, reknitted the last pattern row using a much larger needle and did the crochet cast off again. The edge was still too tight for the shawl to block nicely. More frogging. After a rustle through my stash, I found a tiny ball of Hand Maiden 4ply Cashmere in a brown/black/grey combo and used this to knit the last pattern row and to do a stretchy cast off (K2, *insert left needle into front of stitches on right needle and Ktog, K1; repeat from *).
I'm happy now - more pics when the shawl's blocked. As ever, this may take some time :o)