Well, true ...I forgot about the trait method
Unfortunately the movement points are partly hardcoded and all units start with the same amount which is 80...raising this amount over 100 makes the game buggy and you have to use the trait method.
Nevertheless the trait method is highly complicated and supposes a massive amount of work as this requieres lots of entrys in lots of files to assure that the winter trait is only effective in winter...so every year has to have 4 entrys with the corresponding trait triggers in the export_descr_character_traits.txt to make sure the traits are temporaily and only have effect in either summer or winter...RSII has 638 turns...with 4tpy it is 1276...that means 1276 triggers have to be written just for the export_descr_character_traits.txt...
furthermore this needs additional entrys in the following files export_descr_VnVs_enums, export_VnVs, descr_strat ...not sure I if I forgot one...
comparing the relative low strategic impact on gameplay this is nothing more than a bauble
not really convinced about this... apparently not...there is no script condition check for how much movement points a unit has left neither there is an existing condition check for the season neither there is a console command for give a generic undefined character movement points. For use console_command mp -20 you need to have the specific name of the Character, problem with this is that there is no event/condition check which could check for all characters names who are currently on the map and there would be no way to make the script interactive and use changing character names to reduce or improve movement points...
Instantly I can not think of any possibility to reduce the movement points under certain conditions by script...
The trait method would be the way to do it