I think the idea is cool, but I have a feeling there would be quite a legal battle that ensues when someone is injured or dies in the hostel after they couldn't connect to 911.
Train tunnels, underground car parks and other reception black spots seem to have avoided litigation. Have we really reach the point where going outside to get reception is not something a reasonable person would think of?
None of those are deliberate though.
Just put a pay phone in the back, and make people who stay initial that they acknowledge there is no cellular service.