You properly encrypt it and regularly send data (like in a VPN). Done.
(unless you are thinking about high level things, like "lots of traffic" -> "something going on", but tor won't help with that either)
But is an embassy always only "phoning home"?
(and for really secure stuff you will want a channel with constant rate, constant size packets, as others said)
(Though certainly things like packet timing, packet size, etc. might make more thorough analyses harder to escape…)