The most simple setup (and crudest) would probably be a reverse-acting thermo switch in the cooling water circuit. That would power ON a small 12V "idle-up" solenoid valve when the engine is cold, and OFF once it hits the thermo switch setting.
If you have a standalone ECU with a spare PWM output, you could do a simple PWM idle-up valve controlled according to the water temp sensed by the ECU.
Cheers... jondee86
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