Nathaniel,
"The Wood and Canvas Canoe" does discuss canvasing sponsons, and it is a great read for many reasons. Well worth the purchase. About sponsons, I once had the impression that canvasing them would be a nightmare, but in reality it isn't. Of course it takes more time and materials, but it is not difficult at all. Bottom line- canvasing sponsons is less difficult than and not as much work as a good canvas job on the canoe itself. As always, look to the good folks here for advice. I'd be happy to post photos and discuss the process, as would others I'm sure.
Michael