Bob - Is this for lifting AND supporting or just lifting and putting a log underneath?
Have you thought of using a set of skidding or lifting tongs?
A big log and tongs will require a real tall jack. I don't think my 4 footer would work so well. Otherwise it seems it would work.
What Bob shows would let you jack (blocking as you go) up a good height without resetting the grip on the log.
Tongs, a fulcrum and a long lever handle would be good I think. It would be bulky and heavy but caveman simple. But if you're 10 feet away pulling the lever, slipping the blocks under the log might be tough.