Got it. The trick is to have the library code use an ambient types from a d.ts file (probably nested under a namespace, but you do you), and then have the application code override that type in their own d.ts file.

Library:

// library-namespace.d.ts
namespace MyLibrary {
  type EventName: string; // we don't care in the lib.  String is fine.
}

Application:

// library-overrides.d.ts
namespace MyLibrary {
  type EventName: 'This' | 'That'; // app-specific event types
}