2.8 Can't edit curtin files inside maas snap

As in the title, I know why it can't as SNAP won't allow you to tamper files, but how you can then do it if you want to add few extra commands into your deployment, per documentation you can do it with curtin_userdata https://maas.io/docs/custom-node-setup-preseed , it's been easy in the past as MAAS was installed on the system rather inside the snap.