dropping a signal voltage proportional to signal
This should work fine, but with one gotcha.

Basically if the potentiometer in the ECU has quite a high resistance, then your potential divider circuit (R1+R2) would load the output of this, causing the voltage to drop. In this scenario, the behaviour of the potentiometer would become roughly logarithmic which could make it much harder to control - at one end, tiny movements will translate into larger voltage changes than at the other end which may be very undesirable.

A way around this loading effect would be to use larger resistors for R1 and R2 - if you make sure that they are larger than the overall resistance of the potentiometer, it will reduce the loading. You could also use an op-amp to buffer the signal prior to your resistors, or even use it to do the attenuation.