Evan - I think you hit on the real key in your 2nd point "If you hit the greens, you score fine. The ratio of greens hit by scratch players versus the 5 HCP on the Lake, I would bet is much higher than on most other courses."... which is why I think most courses with a par of 70 are especially good at this. Obviously there are plenty 72's out there that do the same, but I feel the two extra 2-shot holes make consistent ball striking even harder and there are fewer par 5's to take advantage of and make birdies. The more holes that require long iron approaches and tricky greens, the bigger the separation you will see.
My home course, Quaker Ridge is very good at separating scratch players from the 5 handicaps. With OB on the first 8 holes, one stray drive and you are already in trouble with limited birdies chances. Also with the first hole as a par-5, it is extra difficult to come charging out of the gate. Our green contours also don't make it any easier. Most scratch players can put mid-70 rounds together, but the 5 is happy just to break 80.
Other courses I would put on this list are:
Bethpage Black (71) and The Ocean Course (72) not because of length, but because of the premium on course management. You need to have total control of your game and know when to take a chance over the diagonal hazards/forced carries, and when to lay back. Also the weather can be of particular importance at these courses, placing a real premium on club selection. I expect most 5's to keep their ball out of the tall grass (BP) and water/waste areas (OC), but there is a difference between keeping it in play, and having it at the right angle for approach.
I would also give both WF courses a nod here because of the push-up greens and big green contours.