This self-tightening method is based upon a one-way ratchet-like device and a heavy object to pull the slack out.

• Time-lock device + Special Ring Device (SRD)
  Any time-lock can be used. Most reliable are the ice-based padlocks. Attach the time-lock with the SRD on one end to a stationary object near the ceiling. In the example below I used the DIY Ice timelock.
• Weight
  You can use any object heavy enough to pull a rope through the SRD and pull out the slack. I used a ~5kg suitcase.
• Pull rope
  One end of the pull rope is attached to the weight, the other one goes through the SRD to, for example, a cinch noose.
• Trigger rope
  When you are ready you undo a simple show lace know by pulling the trigger rope, what releases the weight. The weight falls down and pull the pull rope tightening the cinch noose.
• Safety rope
  The safety rope prevents the weight from crashing against the floor.

When the ice melts down the lock releases the SRD and the pull rope releasing you or creating enough slack to get to a knife to cut up the ropes.

Here's an example:

The time lock is attached to staircase bars. So it hangs between two floors.

A suitcase used as the weight and pull rope with a cinch noose

The trigger rope. It keeps the suitcase in "midair"

The loose end of the trigger rope is taped in an easy reachable place.

The pull, trigger and safety ropes and the weight.

The suitcase hanging on the safety rope.

The proof of concept is here.