My guess would be that no, you cannot "standby" for free on any ticket for which this is not an advertised and integral feature. And it is not an integral part of an A3 reward ticket.
That you are travelling at an "undesirable time" illustrates why European airlines tend not to allow standbys; it allows them to price each flight and seat separately, and maximise revenue. Otherwise, people will book the cheapest available ticket on the day they wish to travel (on the "undesirable" flight) and have a reasonable expectation of travelling on a more "desirable" flight, for which prices are higher.
The exception may be if the flight for which you have a reservation is overbooked or may be subject to some kind of disruption; in that case, they may be more than happy to move you to a different flight, space permitting, in breach of the normal rules. However, it is far more likely that the "desirable" flight is overbooked than the "undesirable" one, so it's far less likely to occur in your circumstances (where you want to move from the unpopular flight to the popular flight).