I just had an idea that is a shot in the dark but you could try cold calling large companies with complicated SDKs to see if they will pay you a large monthly fee in exchange for exercism hosting a tutorial they create for their SDK.

You already have the sandbox and the support for the most popular languages.

It may take some development to build out the feature but you probably wouldn't need too many to sign on to cover your costs and you could still leave your core offering free.