I'm thinking if you were to put in a metal hatchway and stairs to a lower secured door in the room from outside the garage, you may be better off.
You would use it more, I would anyway as a root cellar storage.
The other issue I can think of is when the car is parked over the door ,sliding or not, how do you get in? You would have to move the car in an emergency when you needed to use the shelter in a hurry.
Then you have all that garage on your door in a collapse and it may be a while before you get out.
Also make sure there is a drain for any water to leave in case this fills up like a swimming pool.
I have done many prepared emergency drills and safety training for just such scenario's. Being prepared is the other half of the battle,the shelter is the start.
We should post something for being prepared and what folks do in emergencies....think I will start a post.
I hope I have helped, though I didnt answer your question.