Have we ever observed cortex being rewired for drastically dissimilar purposes? For example, motor cortex receiving sensory input?

I'm not sure - I have a vague memory of something along those lines but .. nothing specific.

From what I remember, motor, sensor, and association cortex do have some intrinsic differences at the microcircuit level. For example some motor cortex has larger pyramidal cells in the output layer. However, I believe most motor cortex is best described as sensorimotor - it depends heavily on sensor data from the body.

a lot of the initial configuration is prenatal and follows kind of a "script" that might be determined by either some genome-encoded fractal rule

Well yes - there is a general script for the overall architecture, and alot of innate functionality as well, especially in specific regions like the brainstem's pattern generators. As I said in the article - there is always room for innate functionality in the architectural prior and in specific circuits - the brain is certainly not a pure ULM.

Either way, I'm not yet convinced there's a strong argument that all brain function can be explained as working like a ULM (Even if a lot of it can)

ULM refers to the overall architecture, with the general learning part specifically implemented by the distributed BG/cortex/cerbellum modules. But the BG and hippocampal system also rely heavily on learning internally, as does the amygdala and .. probably almost all of it to varying degrees. The brainstem is specifically the place where we can point and say - this is mostly innate circuitry, but even it probably has some learning going on.