I need to automatically mirror a large amount (terabytes) of files in two unix machines over a slow link (1 Mbps). This needs to be done frequently, but the data doesn't change too much (delta transmission doesn't saturate the link).

The usual solution would be rsync, but there's an additional requirement: it's undesirable, from a security standpoint, that either the source or destination machines have (keyless) ssh keys to each other, or any kind of filesystem access. All communication between the two machines should thus be initialized (and mediated) through a third machine.

I've asked a separate question about rsync in particular here. Are there other obvious solutions I'm missing?