createImageBitmap alternative on Safari

Is there some other way to turn a byte array into something you can
feed to drawImage?

You can post the ArrayBuffer of Uint8ClampedArray object to main thread; at main thread substitute using .putImageData() for .drawImage(). As indicated by @Kaiido, it is not necessary to create an ImageData object at Worker

var imgBytes = new Uint8ClampedArray(buffer, offset);
postMessage(imgBytes.buffer, [imgBytes.buffer]);

at main thread

worker.onmessage = function(e) {
  console.log(e.data); // `ArrayBuffer`
  ctx.putImageData(new ImageData(new Uint8ClampedArray(e.data), width, height), 0, 0);
}

http://plnkr.co/edit/N0v1YQHQX2rdFfHcOKeR?p=preview