Just to drag this thread back up again, whee!
Swiss cheese method will help to some extent, but I think also fluting/turning down the barrel would be a significant gain. I'd prefer fluting, but turning is probably slightly easier to do. Both would be a real pain to accomplish w/o machining equipment. Of course, plastic everything goes without saying. I don't think a titanium stamped receiver would hold up.
Titanium isn't stronger than steel for a given volume, it's stronger than steel for a given WEIGHT. So a milled titanium receiver might work, but a stamped probably wouldn't. Even with the milled, you might want a hardened steel insert to brace against stretching and torsioning, and also at the holes for the stock and barrel.
it's be possible to reconfigure the AK to actually use the buffer tube, but I don't see a weight savings there. A buffer tube assembly isn't going to weigh less than a rear trunion.
Oh yeah...and don't make fun of Stephen Hawking!
MC Hawking don't take no lip and he'll bust a cap in you!