John,
In my mind, the half shot penalty would be a bunker just deep enough and steep enough to make getting out of it with a club that will reach the green a 50% proposition.
Although the free form nature of bunkers makes a formula impossible to carry out, if the general ratio from the middle of the bunker to the top lip approximates the loft of the typical club, as shown below, clearing the lip would not be a given. In looking at that list, remember that % slope is about half degree of loft, and the ball launches a bit higher than actual loft, unless hit thin. In general, this would make fw bunkers closer to the green deeper than ones further out.
2 18
3 21
4 24
5 27
6 30.5
7 34
8 38
9 42
AW 46
PW 50
SW 54
LW 58
Of course, I agree with your varitey theory. There is nothing wrong, IMHO, with the occaisonal bunker that you can't recover all the way to the green from, esp. on par 5 holes where the penalty is hitting it in regulation figures (boo hoo) rather than in two shots. Knowing which bunkers to avoid and which to challenge based on probable penalty is part of the strategy.
I did lose a remodel job once for suggesting such a thing, though, so many golfers apparently believe that sand hazards should be no hazard at all. I know clubs that maintain fw bunkers firmer than greenside bunkers so that you can hit a normal shot out, just like you weren't in a bunker at all!
I have also heard the theory that fw bunkers should be simply shaped, both to make it "fairer" about who goes in (no squiggly lines that might have one golfer go in and another on turf) and also that capes and bays treat golfers unfairly, as one may not be blocked by a cape, while another would. If the front edge is simple, and the extravagant shaping is at the back (away from the fw) then I think the worse shot is usually punished more than the one closer to the fw, but not every one sees it this way......
All of which is a long winded way of saying it appears that the modern country club would trend towards the zero penalty bunker.