Also I see articles like this one about "cut-and-come-again" harvesting of loose-leafed lettuce, in which you just chop off the top portion of the lettuce head and leave it and it'll grow back from the base. This seems rather consistent with the ability to grow back again from the stump, so I'm starting to see a pattern here.
And it sure seems like it would let you get more lettuce from less space, if this approach works - I strongly suspect that a lettuce stump with an established root system would grow back to harvest-able size faster than a new head would grow from seed, right?
So just tried it. Went outside, whacked off the top half of the three lettuce heads that are currently big enough to harvest, and made a nice salad (yum!).
We'll see what happens - whether it grows back, and how long it takes. I'll be sure to report back here!