A Heathrow airplane loader was very happy some 10-15 years ago that he was able to reach the cabin from the cargo hold of a plane bound for the West Indies when his colleagues didn’t notice that he was still in there and closed the door. His employer wasn’t as happy, though, when they were presented with an invoice from British Airways for a round ticket to Trinidad (or wherever the plane was headed).
Of course snakes are cold-blooded. A main reason we need oxygen is to burn the calories that keep us warm, which is why we eat a lot - while snakes eat a lot less often. So I assume they also use a lot less oxygen and would take longer to feel the effects of low oxygen… So I assume, as others pointed out, the snakes will slow down due to cold. Meanwhile, they will seek out warm… you.
Legendarily, the cargo compartment temperature control in the cockpit is called the Dead Dog Switch.
I would be sorely disappointed to learn that the scene in The High and the Mighty where John Wayne goes into the belly of the crippled DC-4 to get luggage to throw overboard couldn’t really have happened. 
The really fun “secret” route on the 777 is the escape hatch from the aft overhead crew rest area (bunks and private area for the crew). One of the stowage bins above the passenger seats is a dummy (locked on the outside) that is really an emergency escape from the crew rest. An attendant could, from inside the crew rest, open the stowbin door, flip out a little ladder, and climb down right into the lap of a passenger 
“Don’t mind me. sir, I’m just a carry-on”