Not a bad idea, but it wouldn’t work. First, lining it up properly within the gushing flow at the top of the well would be almost impossible. Second, anything with enough weight to stem the flow would also crack the well head, resulting in a worse leak around the area of the plug.
Keep working on it, though. Not a bad first crack at the problem.
The saw seems to be jammed from the weight of the riser above it pinching it in the kerf. Can they wrap a shaped charge around it just above where the want to cut it off clean and shear that off and get it out of the way?