I very much doubt many package publishers will do this. Most users (if not all) that ask for TypeScript definition files support are doing so because they want

I very much doubt many package publishers will do this. Most users (if not all) that ask for TypeScript definition files support are doing so because they want to take advantage of DefinitelyTyped / @types going forward. However, any tools that can help us write once and use in both Flow and TypeScript will definitely be well received.