The rubberized canvas construction is very similar to that used in white water rafts (uerathane or PVC coated fabric) so I would think that a similar repair would work. Here how the I would make a repair on the raft...

1. Clean the area with a solvent such as MEK or toululene, and remove any ragged pieces.
2. Get some patch material (most raft stores sell 4" wide rolls of material for about $6, you want the PVC stuff) Cut a patch that overlaps the edges by 1", round the corners to help prevent peeling.
3. Roughen the mating surfaces with some 60 grit sandpaper.
4. Use a heavy duty contact adhesive for the repair, such as 'Barge Cement'. Coat both surfaces and let it dry completly. Apply a second coat and let it dry until not longer tacky. Press both surfaces together for a permanant bond. Work the patch firmly into the adhesive with a roller or by pounding with a rubber mallet.

A good patch made on the outside surface should do, after all it works on the white water rafts which are also under pressure. Go ahead and patch both sides if you feel the need, but the inner patch might change the fit of the boots.