Ahh okay yeh there is a non-public API (our GraphQL API - we mention it on the community forum but it’s not technically a public API so there’s a chance of breaking changes) for orgs but it’s not possible to create a linked org via the API.
What exactly are you trying to achieve? Lots of customers of ours are using a Sprite-per-customer sort of setup, so I’m curious what you’re wanting from “sub accounts”?
And what are you imagining is the benefit of a sub account vs a totally separate org? If you want to have each “sub account” have different billing details, that’s just totally separate orgs in Fly.io platform terms.