This is a great question. I was just reading an excerpt in a book that I have had for quite a few years, and it answers some of the questions. It states: (In Canada, it is legal to use euthanized pets in pet food products produced by rendering plants. However, rendering of companion animals in Canada is on a much smaller scale than the practice in the United States. But nonetheless the practice is still legal in Canada too.) The name of the book I am quoting from is "Food Pets Die For" and the author is Ann N. Martin of London, Ontario. She has done a lot of research into the pet food industry secretive operations and the art of keeping the truth away from John Doe public in respects to what goes into our pet foods. From my point of view, this book is a must read for anyone who has a companion animal and has any respect for what they feed it.
It will be interesting to see what you receive as a reply from Delmonte in regards to your question. Knowing the way the system is set up I would expect them to deny it. They only have to tell you what they have put in the pet food themselves. If they have purchased a finished product from a rendering plant (animal fat) all they have to do is mention the finished product. They will play ignorant to knowing what's in the product, or what the other manufacturer is using to make the product. That's basically what happened to Menu Foods a few years ago when the Chinese producer added melamine to a product in order to get a higher protein level in the product for Menu Foods.
I am enclosing a website that has an excerpt from the book "FOOD PETS DIE FOR". It's worth the read, and personally I would suggest getting the book. If you read the book, you will really see the problems she had getting information, and how secretive the industry is.
I have the 2nd edition, and there is a 3rd edition that Ann has published in, I think 2008, which I plan on purchasing, to see what's new.
Hope this helps or sheds a wee bit of light on this very important subject.