Great! Thanks for the info Zutefisk, and thanks to all the other members for their input as well. I've decided to leap in with a 2-part lightweight epoxy filler made by systemthree (
www.systemthree.com) called Quikfair. The company is located in Auburn, WA, but I was able to find their products in Vancouver BC at Fiber-Tek.
The fairing putty, as they call it, has microballoons to keep it lightweight. I slightly bungled the beginning of the first coat by mixing too much epoxy for my first application - the epoxy in my container started to harden after 15 minutes and I got a few chunky bits on the putty knife scoring my coat, which I was also applying too thickly. After that, I only mixed about a cup at a time and had no problems with the application, using a 3" metal flexible putty knife, and working from the centreline down towards the gunwhales, i.e. following the curve of the hull rather than working lengthwise, which was another mistake I'd made at first. Like any process, it takes a bit of practise to get it right, but it wasn't difficult. The 1.5 quart kit would be just about enough for one coat of a 16' canoe, possibly exactly enough if I hadn't lost about a cup right off the bat due to mixing too much. Once mixed, the consistency is viscous enough to be spread easily, and it doesn't drip or run at all. I think with the smaller batches you get a bit more working time as well, as I suspect a larger batch reaches some chemical critical mass faster, although that's just a guess. Fifteen minutes is plenty of time to carefully apply a cup of the mixture, and a cup goes pretty far. It's ready to sand in three hours, and sands very easily by hand - except where I botched that first bit, which may require a power tool - or just a lot of hand sanding and a careful second coat. I'm thinking two thin coats will do it. Even after one thin coat and sanding, the canvas feels pretty smooth, and it definitely has a nice hardness to it. As these products are made for boatbuilders, I'm anticipating that the Quikfair will be flexible enough to not crack, but time will tell. I'll post photos and let you know how it turns out. I'm a complete amateur at this, so cross your fingers!