I don't think a carbon pile would be particularly stable, especially as it heats up. You could include it in a feedback loop and use an electromagnet to vary the pressure so as to keep it stable, but that would involve some way of monitoring its resistance.
For low resistance high current purposes, big wirewound resistors with multiple tappings are the cheapest and most stable answer. They are relatively easy to construct from slate bars and resistance wire, but the tapping switch is going to be the most difficult item to design and make.
If you want to go down this route, please ask for more information. I have made several 5 kW* units as replacements in motor-starting applications, so I may be able to help.
~ Adrian Tuddenham ~
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