Never try to engrave through any type of plating. Period.
It is best to either choose another canvas and leave the plated piece alone but if you must, there are products that Brownell's sells that will remove nickel plating. I don't know which it works on, electroplating or electroless nickel plating or both.
If it's chrome by any chance you can take that to a chrome shop and let them hang it in their de-chroming tank a while and it'll come right off but will probably matte the metal so you'll have to smoothe it back up.
actually, you can engrave through nickel, general expereince indicates that it won't peel like chrome will. But I would only do that for something like initials or a simple logo. The burrs you raise from engraving need polished off afterwards, and doing that will ruin the polsih of the plated material