Can the skin / copper oxide / copper junction of an old exposed mains conductor produce a safety-significant DC component to a shock?
Copper oxide does introduce measurable nonlinearities - so your concern is warranted.
However, we can quickly, from systems where there's a lot of AC voltage intentionally brought onto copper parts (namely, broadcast and cellular transmitter stations), gather from the severity of the PIM (passive intermodulation) that the energy in these rectified parts isn't severe enough to be harmful to someone touching a piece of conductor connected to an &quot;accidental copper diode&quot; where not 1 kW of RF is pumped through...
Another aspect: these diode junctions have breakdown voltages in the region of 1 V. By itself, a diode of that type can't sustain a potential difference greater than 1 V; so no risk. There would have to be a lot of factors coming together (like... an old-school metal-oxide layered disk stack diode) to yield dangerous voltages.
You can safely ignore this, and worry more about other occupational hazards.