cool idea! how are you locking out the apps? my first thought is the SYSTEM_ALERT_WINDOW permission which lets you draw over other apps but maybe theres a better way to do this in the new APIs.
Thanks! It's quite simple actually - what I do is monitoring the current app in the foreground with a background service and when the package name (unique to each app) matches the one in the app database, StepLock fires up.