Thanks for the help everyone. I'm concluding that the bicycle is capable of accepting a 26.8 diameter seatpost (confirmed last night) but would benefit from a reaming to do it (as it was helpfully explained). Moreover, there are a handful of less trustworthy examples that arrive at the same conclusion, including Sheldon's database. Since the 26.6 post moves well enough in the tube, I'm going to conclude that that's the intended size!