Create a workflow Business Process that runs an SDK script locking the Starting entry. Or have workflow trigger based on a metadata field change (Like "Locked"="Yes/No").

Relevant code snippet below. When and how you call Unlock() depends on how the customer wants to lift the lock.

using (DocumentInfo docInfo = (DocumentInfo)this.BoundEntryInfo){
	docInfo.Lock(LockType.Exclusive);
	#docInfo.Unlock();
}

Update: this approach probably won't work for what you want because the lock won't persist between script instances. Having Workflow Check-Out and later Check-In the entry is a more viable route as it will create a persistent lock between those events.