What are you defining as a lead blocker?
is it a wide receiver that happens to be leading a sweep, or is it a lineman or a back... The answer is a little different for each group.
generally speaking, an offensive player can block below the waist anywhere on the field as long as he doesn't violate one of the numerous caveats to the blocking below the waist rules.
IE, if he is unrestricted at the snap, he can block low anywhere on the field prior to a change of possession, as long as he is not past the neutral zone blocking below the waist at any angle back toward his goal line.... if he is restricted at the snap, he can only block below the waist N/S or toward his adjacent sideline if he is beyond the neutral zone, and he cannot block below the waist in any direction in the tackle box, or, same as unrestricted, back toward his goal line if the block is beyond the NZ.