Append with cloud-init write_files

per https://cloudinit.readthedocs.io/en/latest/topics/modules.html#write-files

write_files:
  - path: /home/user/some-file
    content: | 
       Line to append!
    append: true

It was not possible, but I added the functionality and raised a merge request which was subsequently approved.

The functionality is available from 18.5 onwards.