This isn't possible.
Minecraft solely supports ONE 3D-8edged box for any sort of rendering, collision detection and hitbox display.
However, within the rendering engine you can overcome this by (hell, it's a crappy solution, but that's what notch did) changing the whole box of ALL blocks of a type for a ms, then render the block, change it again, render again, etc, then change it back to normal. This way you can render irregular forms.
However, this does solely work for rendering afaik.
If i remember correctly, SpaceToads pipe can display a "correct" markBox, too, but if you attempt to place a block the actual hitbox is used... thus, doing it like in BC would merely cause the hitbox not to be displayed as "cube" anymore and lead to (eventually fatal (HV!)) missplacing of cables.