You just need something to replace the 14oz tomatoes called for. I'm not sure what the mock tomato sauce you were asking about is, but don't know why that wouldn't work.
If you tolerate roasted red bell peppers (the kind you can buy bottled) you could puree some of those with liquid to replace the tomatoes too.
I'm actually betting you could just omit the tomatoes and not even replace them, and not notice much difference. Worst case is the soup looks a little too thick, and you have to add more liquid of some kind.
Soups are nice and forgiving that way!
- H
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