No, not a hem, though it was a possiblity; I thought about. Instead I opted for a border-- Simple garter stitch. There is plenty in the yoke on top, and the button bands are garter, so a garter stitch border is just the right touch.
I ended up with 6+inches of the mock smocking, and now with 6 garter ridges for the border, the final measure will be 7 inches below the armhole, and 12 inches from back neck to lower edge. A nice length.
I still need a few more rows to finish the border, and then the sleeves, and then the sweaters will be done. There will be enough yarn left over for a pair of hats, and likely even a pair of booties.