Hi Maciej,
I think I will vote yes, if we could fix this issue without modify ExternalActviityBehaviour. We still have other way to implement this feature. e.g. modify TaskActivity to notify if there are out of date tasks should be deleted.
When you check the patch and testcase, please make attention this comment of Ronald in TimerImpl:
// I am not sure here, timeout() just needs to be called in case we are
// executing for a task activity
ActivityImpl activity = execution.getActivity();
ExternalActivityBehaviour behaviour = (ExternalActivityBehaviour) activity.getActivityBehaviour();
behaviour.timeout(execution, signalName);
So I will try to find a solution without changed public API. Then we could discuss about which solution could be taken.