The M307 I (invert) parameter may do what you want. However, RRF normally considers any temperature below 40C as off/don't care, so I suspect that firmware changes would be needed too. Caution: using this parameter might cause the element to be on at full power when you don't expect it, so test it with care.
Note, peltier elements do not work efficiently with raw PWM. To work well, they need smoothed PWM, which requires a reasonably high PWM frequency, flyback diode, inductor and optionally a capacitor. OTOH, if you need to run the element in bang-bang mode, or at or close to full PWM most of the time, that may not matter.
