Roger,
I doubt if Thompson's would be a good sealer. It's more akin to diesel fuel than a good sealer (sure smells like diesel anyway :-) How about Mike's Varathane treatment. It ought to work on canvas. May take a few coats, though. Also you might consider dying the canvas before covering to reduce your bright white problem. Rit dyes would do the trick. I considered this for a while, but opted for the roof paint approach.

Bill