No. They injected code into some visitor analytics script (like GA) and those scripts are constantly updating so you cannot calculate and store the hash.

Nope: http://www.jefftk.com/p/sri-against-the-great-cannon

I guess so. GitHub hit by DDoS attack https://news.ycombinator.com/item?id=9275041

Only if this feature had widespread adoption. And if the GFW doesn't replace the hashes in the original content.