It's not a dumb question at all. I spent a lot of time researching the Malnati's pizza before and, although I once lived for a brief time in the Chicago area, I do not recall having a Malnati's deep-dish pizza.
I only ask because it strikes me that you might be at a disadvantage trying to match a distinct taste of tomatoes to something you hadn't tried before. Although, I appreciate the careful deductions of possible places he may have gone approach.
I've had Malnati's twice, and the second time was because the first time was so damn good. The three things that stood out the most for me were the crust, the sausage, and the tomatoes. Perhaps most distinctly different than anything else I'd tried were the tomatoes. They didn't taste canned. They tasted fresh, zesty, and a little sweet, like heirloom organic sweet. They were a tomato's tomato.
Everything I've tried since then hasn't really come close to matching them. As you know, he mentioned sending guys out to California to get fresh "vine-ripened plum tomatoes." Given the mounting list of distributors in California, I haven't had much luck getting my hands on them and certainly there's nothing saying that I ever will, as his partnership was likely proprietary.
One thing worth noting is that California is home to several disease vectors affecting tomatoes, specifically the beet leafhopper (I learned this the hard way trying to grow a variety of hot peppers, and some heirloom tomatoes organically in Northern California). Subsequently disease-resistant hybrid cultivars are preferred among local growers (although I'm not sure any hybridization has immunized any tomatoes from curly-top virus yet) That narrows the field some I think, but the search continues.