As always, I love your Pixel work, Mike.
I must admit, I have never thought of or seen the Clever method for Iso displays. From a design standpoint, that is an extremely crafty way of depicting Iso graphics without obscuring tiles.
Even so, I think the Clean Alternative (Alter) is my favorite: it has the same obfuscation as the Clean method, but the walls look more solid. Beyond that, natural solid tiles that project into a chamber (for instance, rough-walled stone caves, etc.) would look like thin walls using the Clean method. Also, if viewing a wall from the Top-Left in Adventure Mode, the entire square (including the floor underneath the thin wall that would be out of sight) would be lit up as though in Line-of-Sight, which would look strange.
The only concern would be that semi-obscured sprites (such as Dwarves, or the unit of Meat in the example) would be visible and recognizable. This could be mitigated by using careful sprite design, partial cut-aways for the foremost wall tiles, and/or 90° display rotation.
I'm really excited by the prospect of a real graphical representation of DF like this... such Graphics are entirely feasible, and would look really sharp and intuitive. I'm keeping my fingers crossed that DF modders/hackers realize how feasible it would be to implement such a project... and moreover that Toady begins to weigh the intuitiveness of a good Iso graphical display against the obscure and exclusive nature of ASCII graphics and keyboard-only interfaces.
EDIT:
On the subject of Isometric Interfaces:
http://www.bay12games.com/forum/index.php?topic=35050.msg537495#msg537495