There are 4 calls you can include using X-Plane's REGION_DREF command:
maxxxp/seasonsxp/is_summer
maxxxp/seasonsxp/is_autumn
maxxxp/seasonsxp/is_mid_winter
maxxxp/seasonsxp/is_winter
For example, if you have autumn-specific scenery in your package, you can include:
REGION_DREF maxxxp/seasonsxp/is_autumn == 1
This is all structured in the library.txt. An airport scenery package will now need a separate folder for each seasonal variation, most likely just three since you can exclude summer if you so chose. Ideally you can edit your textures, to include orthos, place them in the appropriate folder and call from the library.txt to present them. Or not ideally, but already built into TM, is using the lego brick assets (objects not included). When a seasonal variant is selected in TM it will also call from your scenery automatically. All this will make WAAAAYYYY more sense when I have the tutorial put together.......