Personally, I would write Lennox directly and politely explain that you should not be out $500 to replace a defective heat exchanger which was caused SOLELY by an admittedly defective manufacturing procedure. What separated Lennox from the competition at the time was the extremely high efficiency (95%) and the 20 year (later lifetime) heat exchanger warranty. Again, this unit did not fail due to normal wear and tear, but solely because of a manufacturing process that they used and approved. Later, when it was discovered that huge numbers of these were affected, along with the remote possibility of carbon monoxide poisoning, (they sent out free CO detectors to those customers that called in their model/serial #'s) they offered to replace the heat exchangers or give a discount on a new non-pulse furnace. They replaced mine at no charge. Given the positive image that Lennox tries to project, plus the liability concerns with a customer operating a known leaking furnace, I would think that replacing this unit would be a no-brainer. You should not have to pay a dime to replace an admittedly defective product.
Mine was replaced about 2 years ago, and has had no problems since. As I mentioned, they beefed up the design, and the welds looked much better than the original. I have put an hour meter and starts counter (triggered by the gas valve signal) on the furnace last winter, so I will have at least a ballpark number of how many fired hours/starts it takes to fail it if it ever develops another leak. I plan to keep this unit running for as long as possible. Aside from the combustion noise, it is a good safe design. All combustion air is taken from outside, and if for any reason it fails to ignite or the gas valve sticks open, the gas will automatically be vented outside, with no possiblility of blowing up your house.
PS: I don't know what the maximum small claims court dollar figure is in your state, but $500 is probably within it. I would use this as a last resort, but you probably won't have to if you contact Lennox.