I prefer this dead man practice mode idea, because new players can only practicing (e.g. learning jumps/jumping) on maps that where played. A small jail don't have lots of possibilities to practice in.
Adding "forcejail" "1" to the worldspawn should avoid that people can turn the jail mode off, but most of the maps with a jail don't use this. I added it to the .ent files I started to create last year, not sure if jitspoe sets up a central repository for .ent files sometime, though.