Seems all right to me. The S12X is qualified Automotive so you have functionalities guaranteed from -40°C up to +125°C.
In this case you won't have zillions of cycles for Flash but your application doesn't seem to require them anyway, so sounds good to me.
If you go lower in temperature, it won't be guaranteed so I would make sure you have a very good oscillator circuitry (so it always starts) and that you are careful when programing flash (especially if you intend to program at lower than -40°C), implement checksums or things like this.
Anybody else thinks about something ?