one COULD technically achieve this to some extent, using the following method:
all mcds (map cell datas?) have 3 TU values: walk, fly and slide. snakemen are the only units in either ufo or tftd that use the slide value. increase these values in polar/mountain terrain and reduce them in desert, and done!
it wouldn't affect their reactions so much, since they reserve TUs a lot of the time, but it would definitely hinder (or enhance) their movement capabilities.