-snip-
I'm reading the OP's question in a different way. He wants to ensure the private key has been written to disk so that if there is sudden power loss you don't loose bitcoins. Sort of an atomic operation. I don't think he's worried about some malware reading the keys from memory. If you have malware it could just as easily read the keys from disk.
(Also, electrum is coded in python not java. Your point about lack of low level hardware access stands, though.)
You are probably right, now that I read it again "write behind cache" makes more sense if we are talking about writing to HDD.
But wouldnt this only be relevant after creating a new private key? So If I want to be paranoid about that a reboot would indeed be a solution. Open wallet -> create new private key and address -> close wallet -> reboot system -> open wallet and use the newly generated address. Repeat if you need a new address or generate a sufficient number of addresses the first time.