Embedding observable framework in another express app

[djMax]

I would like to embed observable in an existing express app. I generally have things working properly in "production" but I'm having a hard time integrating observable preview properly. I assume this is because it doesn't output files to the build directory (perfectly reasonable). Is there a way to host observable preview inside express?

[djMax]

I have made something that seemingly works, by doing this:

// observable.d.ts
declare module '@observablehq/framework/dist/preview.js' {
  import type { IncomingMessage, Server, ServerResponse } from 'http';
  declare class PreviewServer {
    constructor(args: {
      server: Server;
    });
    _handleRequest(req: IncomingMessage, res: ServerResponse): void;
  }

  export { PreviewServer };
}

export async function attachObservablePreview(server: Server) {
  const { PreviewServer } = await import('@observablehq/framework/dist/preview.js');
  const observablePreview = new PreviewServer({
    server: Object.create(server, {
      on: {
        value(event: string, listener: () => unknown) {
          // Ignore request event listeners because we will handle those out of band
          if (event === 'request') {
            return this;
          }
          return server.on(event, listener);
        },
      },
    }),
  });
  app.get(/.*/, observablePreview._handleRequest);
}