Disclaimer: I'm not familiar with backing up Lotus Domino servers.
However, despite that, I would jut like to throw out there than 12.5 works with at least back to 10d's remote agents (for the purpose of rolling upgrades). Technically, all of your agents are supposed to match the version of your BE server, but I've never seen this cause the backup to fail. It will typically just give you the "Completed with exceptions: please upgrade the remote agent on this server" message.