I think I did it.

client/driver/driver.go

@@ -293,10 +293,10 @@ func GetTaskEnv(taskDir *allocdir.TaskDir, node *structs.Node,
 	env := env.NewTaskEnvironment(node).
 		SetTaskMeta(alloc.Job.CombinedTaskMeta(alloc.TaskGroup, task.Name)).
 		SetJobName(alloc.Job.Name).
+		SetDatacenterName(node.Datacenter).
+		SetRegionName(node.Region).
 		SetEnvvars(task.Env).
 		SetTaskName(task.Name)
-		SetDatacenterName(node.Datacenter)
-		SetRegionName(node.Region)
 
 	// Vary paths by filesystem isolation used
 	drv, err := NewDriver(task.Driver, NewEmptyDriverContext())

client/driver/env/env.go

@@ -520,22 +520,22 @@ func (t *TaskEnvironment) ClearJobName() *TaskEnvironment {
 }
 
 func (t *TaskEnvironment) SetDatacenterName(name string) *TaskEnvironment {
-	t.JobName = name
+	t.Datacenter = name
 	return t
 }
 
 func (t *TaskEnvironment) ClearDatacenterName() *TaskEnvironment {
-	t.JobName = ""
+	t.Datacenter = ""
 	return t
 }
 
 func (t *TaskEnvironment) SetRegionName(name string) *TaskEnvironment {
-	t.JobName = name
+	t.Region = name
 	return t
 }
 
 func (t *TaskEnvironment) ClearRegionName() *TaskEnvironment {
-	t.JobName = ""
+	t.Region = ""
 	return t
 }
 

command/agent/agent.go

@@ -280,6 +280,7 @@ func (a *Agent) clientConfig() (*clientconfig.Config, error) {
 	// Setup the node
 	conf.Node = new(structs.Node)
 	conf.Node.Datacenter = a.config.Datacenter
+	conf.Node.Region = a.config.Region
 	conf.Node.Name = a.config.NodeName
 	conf.Node.Meta = a.config.Client.Meta
 	conf.Node.NodeClass = a.config.Client.NodeClass