Followup. Feisol to the rescue!
Photrio denizen bdial messaged me. He'd found that the Feisol CC-2807 short center column is the same diameter as his Gitzo 1227's center column. I have the 1227MkII, which added an anti-rotation stud in the head with a matching slotted center column, and the Feisol part lacks this slot, but he suggested I simply rest the bottom of the shortie (sans weight hook) on the stud. The top of the stud is about 41mm down from the top of the collar so there's plenty of purchase and I can tighten the collar lock just fine. Obviously I can't use the weight hook (nor can Barry, as it turns out, because the CC-2807 is just a hair too short to allow him to attach it) but that's not really a sacrifice when the tripod's that low as the legs will necessarily be pretty splayed out.
This is an elegant, inexpensive, and lightweight solution that can live in my photo backpack, allowing me to quickly switch over the tripod in the field when needed.
My hat's off to Barry!