You can use custom iso though the api; https://www.99stack.com/resources/docs/v-1.1;
Upload and then follow the instructions for deploying custom iso via the create function, also via api, I don't think you can do it via control panel yet. It should work in all regions.
It's flashed to disk directly tho so it has to be an installed iso, and not just a install medium. But you could run it live too I guess as read only.