Then I assume this wide zoom Sigma 2.8 will work fine on a full frame:
Sigma - Lenses

Will this mylar hack still utilize the auto focus?

-- peer

Yes, that lens will work. However, I've owned a couple of Sigma lenses. Be aware that they let their lenses open wide for a reasonable price, but the performance of their lenses is often poor at wide apertures.

In other words, they offer great specs for the price, but the performance might be no better than a Nikon or Canon f/4 lens. If the corners get soft and dark at f/2.8, it might not be usable out there.

I'm not sure about this particular lens. Read some reviews and pay particular attention to wide-open performance. Note that the example image in the link above was shot at f/5.6.

Unfortunately, no. The inability to control aperture of Canon lenses is a huge problem. We lose not only autofocus, but image stabilization as well when we insert the mylar or untwist the lens. (Untwisting separates the connections, but you risk dropping the lens. Mylar is the safer alternative.)

I can understand Canon not allowing full manual control. At a minimum, they should let us control aperture. That would put them ahead of Nikon lenses as we can then use Canon's photo functions, IS and AF.