Cable marked "welding cable", and listed as such by UL, has been evaluated for use to as high as 100 volts- unless marked with a different (higher) voltage rating. I am not sure as to any sunlight or wet location ratings; it is safe to assume it is not evaluated for direct burial.
I would also assume that, following general trade practices, that you would need to run both cables in the same pipe- whether you had a DC welder, or not.
I do not see any code objections to running the wires in pipe. While they are certainly flexible, and certainly cords, welding cable is a specialty cable, and not a general use cord as are cords such as SO.
