during my first dirty dozen attempt, which ended with me riding along with my friends filming it for a documentary, i watched and later saw video (ok, i found it
here) of a woman sitting and spinning up canton. so yeah, it can absolutely be done. she was on a hybrid with, one imagines, incredibly low gearing.
the reason everyone (including me) stands when going up canton is that we simply don't have the gearing required; standing is a must, in order to generate the downward force to turn the pedals over (while pulling up on the handlebars, and also leaning forward, and trying to maintain balance at near-zero speed, and also trying to pick the perfect line with virtually no control--canton is fun and you should try it sometime!).
the downside of sitting is that it becomes much more difficult to distribute your weight forward enough to maintain traction (just ask @reddan, who as tried it many times on a recumbent). at the same time, though, if you can distribute your weight forward enough to maintain traction, sitting is actually much better than standing because you can maintain a consistency of force on the pedals and thus the rear wheel on the ground.
the hard part of canton, for me, is that it's so difficult to not slip out and lose your momentum. when you're going up that steep a grade, losing your momentum generally means immediately falling over. being able to maintain a constant force on the pedals, such as you might while sitting up a lesser hill that is slippy from ice or leaves or some such, gives a great advantage.
