Electrum, supplemented with a printer interface which blocks until the key has been printed out could work. But I understand that there is no ready-to-use solution yet which automates the process of having keys securely synced before being used on the network.
Printers are even slower than hard disks and they have their own buffers too. The OS maintains a print spool file as well. So there are many levels of buffering. Electrum would be told it's "all good, a-ok, go ahead" but in reality it's still sitting in one of those intermediary places and hasn't yet been printed onto paper.
It's not so different from write-behind caching. Would be the responsibility of the printer driver supplier to properly implement a synchronous printing mode. Anyway, the best hard disk or printer driver won't help if the Bitcoin client doesn't enforce synchronous writing of private keys on its side.