The simple answer to the question is to get a stainless steel inner cable set for brakes and shifter, along with something like upper level Shimano MTB outers and fit them, lubricated with silicon grease, the type used on rubber seals for swimming pool pumps. If you look at MTB cable sets, you probably can get them with outer-cable ends with extended nipples on them. I am not sure if Shimano's sets provide them, but others do. A browse in an LBS would be helpful.

Honestly, the discussion about stainless steel is irrelevant, but typically BF boffin. Just to add to the discussion, in marine circles, it is considered very bad form to take to stainless steel plate with an angle grinder, which produces surface "rusting" or discoloration. Work hardening by bending also can lead to discoloration.