Finally I've found an app that introduces a delayed PIN lock of an Android device. Makes me wonder why is such a simple feature lacking for so long.
[APP][18/6/13]+Smart Lock+V1.2.4+Simple delayed PIN Lock Screen - xda-developers
With Smart Lock, you set up the activation period and, after the screen has been off for that time, a PIN protection kicks in. The protection is provided by the app, it is not using the system PIN security. This implementation is a bit better since the default (system) PIN prompt also requires you to press Enter key, which is quite unnecessary.