Even more frustrating would be reading about people stranded in a blizzard who had everything they needed in the car with them ... but didn't know it was there!

True enough. It still doesn't solve the problem. I was hoping to come up with a third option. Perhaps it doesn't exist.

Let 'em freeze, I guess. I don't like it, but the consensus on this hypothetical question seems to be that nothing can be done.

Maybe a short book that entertains as well as teaches?

Cody Lundin's book "9.86 Degrees" might qualify. If their interest is tweaked, then move on to gear.

Actually, the same dilemma exists all around us. For example, you know for sure that food prices will jump with inflation, and you suggest to loved ones to stockpile a few items, but they don't listen. So, should I jump in their home after midnight like Santa and stash a few cans under their beds or sofas ??

People need to be educated first. As long as they do not accept the idea, there seems no point in hiding something in their pockets.

And there is another point
Why have you chosen a method of SECRETLY adding things in their cars instead of just requesting them to accept the gift and please put in the trunk, just in case . I think this would be a more logical way.

Something can be done. Inform them, teach them and help them equip their car. But if they refuse, then there is nothing yu can do.
You can lead a horse to the river but you can´t force it to drink.