Is there any way to be notified of standby / hibernate system events using Qt ?
If not, is there another way to catch these events ?

Qt has the QSessionManager but this one doesn't provide the means to check for these events.

Qt guidelines tell us that an application doesn't need to do anything on standby / hibernate, but I'm building a simple
chat client which should do a friendly disconnect from the server when going into standby / hibernate and a reconnect
when resuming from standby / hibernate.

Cheers !