I've not used a service such as this before.
If you can ssh into the production servers from the Internet side, then Affa can also work over the 'net. You could set up an offsite Affa server with a large amount of storage and then use it to dial in to each client server in turn to back up.
The first time you backed up the bandwidth use would be quite high, but as subsequent backups are differential it might not be particularly onerous, depending on the amount of data involved.