What do you mean by grave? Can you have the call drop after the night message or do you point it towards a dummy extension or something?
It comes from a common phrase in our industry is "cradle to grave call flow"... grave means it's final destination, a mailbox, extension, hunt group, disconnect, etc... the destination itself is not relevant, as long as it has a final resting (stopping) point where it can't automatically get looped around anymore, the call will "stop" somewhere.
In the embedded voicemail there isn't a lot of options, what you are trying to do cannot be easily/cleanly done to my knowledge... Have you tried recording the message twice in the same greeting?