This is not answering the question, but someone please build this for me (and hopefully a million others): Simple live off-site backup for worst-case scenario.

I buy a pair of devices (e.g. Tonidos) with attached commodity HDDs that I split with a friend. He plugs his into his network, I plug mine in.

I dump files to my local device, but they actually get buffered and then stored overnight at his place, and vice-versa. It's a remote backup storage device, not a NAS. Not for fast file serving.

If my house burns down, he hands me the HDDs and I decrypt them with my password.

If his IP changes, his device re-registers with mine somehow. Cleverness needed here, not a $10 month subscription. E.g. his IP is emailed to me, or my device, or something like that. Someone knows how to make two devices find each other over the net, I am sure.

Edit: Of course, it would be nice if it still worked if he was tech-illiterate, e.g. you could give the other one to your elderly parent or neighbor or whatever.