According to the quick measurement I did when I gave a try to the Surly Knard 29x3.0 it is very close in diameter to a 26x4.8.
That is true that you can ensure crown clearance without the lateral clearance.
With 2 frame versions you are covering everything:
-the narrow one (that fits 26x4.0 / 29x3.0) for the leg-bashing riders
-the wide one (that fits 26x5.0 / 29x3.x) for the paddle-boat riders