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Something along the lines of a circuit that has say 6 transistors set to trip at various input voltages would probably work. Say if you put the voltage from the fanbus into these transistors at 6v and set half of them to turn on. This could then turn on 3 relays.
If you've got each relay in series with a really big resistor (say such that it will turn on with 1 resistor in series - just make sure they can take the current), and wire all the relays up in parallell then increasing the number of relays turned on will reduce the resistance across the control circuit, and hence increase the current through the fan.
If you can be bothered to sit there and work out all the resistor values, you could extend the number of transistors/relays until you get a reasonably linear control system.