I dont know if this issue has already been solved, but how about a non removable "Temperature phial"!? (You'll definately need a better name for that!

)
By placing and removing this item to the players inventory, dependent on the temperature, you'll be able to take a look at some kind of thermometer every time you like!
You could scale it like your hypothermia states:
Warm (Item description: "The phial is all warm!")
The liquid inside the phial has a slightly transparent golden color and is glowing.
Comfortable (Item description: "The phial has a suitable temperature!")
Transparent golden color, without the glowing effect.
Cold (Item description: "The phial is cold!")
The liquid has turned completely transparent.
Freezing (Item description: "The phial is freezing cold!")
The liquid turns to a cold transparent blue.
Freezing to death - Death: (Item description: "The phial is frozen!")
The liquid still has its transparent blue look, but with an added frost effect and some frost can be seen on the phial.
What do you think!?