Unless the canvas is quite thick, adjusting the platen gap shouldn't be necessary.
Getting the canvas to feed will be a bit more difficult. Taping a leader can be hit and miss. So can other methods, for that matter. Drop the canvas into the rear feeder slot. When it hits the bottom, use both hands on the front of the canvas (use gloves if you're worried about oil from your hands affecting the print), positioned down into the slot as far as you can (be careful because if you're too far down, you could get your fingers nipped by the rollers) and apply gentle downward pressure on the canvas to help it catch in the rollers. You may have to try a few times. Works for me and I get the sheets to take up on first try about 80% of the time now. I position my hands backwards so my knuckles are on the paper beause it's a bit more comfortable than turning them palm in.
It'd be nice if these printers could take the full 17" width through the front, straight through path because that would make loading the canvas much easier but, alas, the straight through path only accepts 16" wide stock.