I’ve downloaded Hazel. It doesn’t look like it has the internal capability of appending text to a file, so I think you’d have to use it in conjunction with an Automator script (or Javascript, or shell script) – which is its own can of worms.
Also, the ## New lane can be the last lane, but if you archive kanban notes (as I do), the Archive section of the board’s note will always be at the bottom. So if you append text to the bottom of the file, it will go into the archive.
I’m sure someone with the scripting chops could overcome these hurdles, but alas, I’m not that someone 
Cheers!