If the stock water temp sensor for the fans is a resistor rather than a switch, we could alter the temp when the fans come on by putting a resistor inline or parallel to the sensor's wire. You wouldn't have to use the Miata's lower temp sensor and if you wanted to go really gonzo you could use a pot or a switch for different settings: one for winter and one for summer.

Anyways, just an idea I am going to check out in the manual tonight.

 

Without checking, I thought that all temp sensors were thermocouples with bimetallic elements. when it reaches a certain temperature, the current flows through it. When lower, it is like an open circuit. They sometimes feed a relay to turn fans on and off. I am not specifically sure of the RX7's specifics though. If I am right, all you need to do is get a thermocouple that is set at a lower temperature threshold and install. (This sounds much easier than it probably is though).

Tim