It would be useful if QStateMachine had a signal to notify about configuration changes

Currently there is no centralized mechanism for being notified of state changes; you'll have to do something like inherit QAbstract

, reimplement on

and only use those custom states in your state machine.
If QStateMachine had a signal for this, custom states would not be necessary. The signal could e.g. be used to create a "trace" of the state machine's execution, by recording the configuration at each step.