A big component of pizza sauce has nothing to do with tomatoes per se. Break down the components and consider alternatives.
Water: Yup, tomatoes are just wet, so you'll need some liquid.
Sugar: Tomatoes are naturally sweet, so you'll need some sugar/honey
Acid: This is huge, but easily fudged with vinegar or lemon juice
Seasonings: As much onion (powder), garlic (powder), salt, pepper, basil, oregano, parsley as you want
Depth: This is the tricky part, that rich, almost smoky quality of cooked tomatoes. It won't be easy to replace. Darin's answer probably comes closest with a similarly colored pepper cooked with direct heat. To expand on that, I think you could venture into other peppers besides just red bell, maybe some mexican low-heat varieties, a little chipotle, perhaps?