I have never really seen it done on a Miata however, if you have the time to wire in a wideband o2 sensor I don't see why you couldn't do it. Innovate motorsports makes a digital wideband that you can program to act as a narrow band that has a switching point other than 14.7 to 1.
this is assuming that the car uses a narrow band from the factory. I have never really worked on the Miata, but most Mazdas and most cars in general have a narrow band o2 sensor. I don't see why you couldn't wire in a switch that you could alternating between the two of two sensors. Of course this would have to be a dual position switch. With the switch wired in such a way the signal wires are selectable.
The only problem is if you go too lean you will throw a code for the secondary o2 sensors.
If you go even more lean... kaboom. By-by engine. .. i would never go leaner then say... 17 to 1, 16 would be safer.
Here is a link to the o2 sensor I am using on my track car.
Innovate Motorsports 3844 MTXI WDEBND UEGO GAUG KT
All this being said. I feel like a 5speed would be a more cost effective upgrade.