I would not do it, and I think it is unethical because it involves dishonest behaviour: even though you don't need to provide the wrong serial number (which would be a direct lie) you are still implying that it is the same piece of equipment that you bought. The cost of the warranty is included in the cost of the tablet, and when you lost the tablet you also lost that warranty. Hence you don't have the right to use it, and implying that you do would be unethical.

As I understood it, this was a hypothetical question for an ethics class, not an example from the poster's life.