Sorry for the late reply, I was off this weekend...
You can fix your plugin but it would be a little complicated. To tile a spritesheet frame you need to:
1) make a new canvas and get a 2D context
2) drawImage() the sprite's image on to it (you'll have to see the new code in Sprite's draw() which shows how to draw the part from the spritesheet)
3) make a new Image() and set its src to canvas.toDataURL() - now you have a standalone image of the section from the spritesheet
4) now you can make a pattern of that image and draw the section tiled
I can't immediately think of a way to get this working with WebGL, but you could just turn it off for now.
It's a good point why sprites can't be tiled and why Tiled Background exists at all. But Tiled Background has a slightly lower overhead than Sprite, and in future it may be possible to better optimise Tiled Background based on the fact it's simpler than sprite.