Yes, I wrote one [1] and others did too, e.g. [2] and [3].

The gist of it, as mentioned in [4], is that you need to have a web server that implements checkStatus and saveConfig PUTs, and PUT and DELETE for saveBoard.

[1] https://github.com/apankrat/nullboard-agent

[2] https://github.com/luismedel/nbagent

[3] https://github.com/OfryL/nullboard-nodejs-agent

[4] https://nullboard.io/backups