Something like

message Pool {
  string name;                 // e.g "Chrome"  
  repeated string schedulers;  // groups with whoever is allowed to scheduler tasks there
  string default_task_account; // a service account to use for tasks assigned to this pool by default
}

This will allow most users of various non-main internal Swarming instances to configure their pool with service accounts in one place, and don't bother specifying it everywhere where they post a task.