I'm doing some building with turtles and though they can place Carpenter's Blocks—even slopes, with a little care—they can't texture them. Trying a few ways, I imagined them being able to accept a piped (or placed from a turtle or golem, etc) block to activate the appearance change. The same process could be reversed to remove the block and return the Carpenter's Block to its natural appearance.

I got excited by the idea because it opens up a whole slew of new aesthetic possibilities. One simple example would be a Carpenter's Block in a stone wall that looks like a natural part of the wall. Flick a lever though, and the stone inside it is pulled out and replaced by a block of glowstone.

Beyond construction automation, I can think of a lot of ways this sort of automated appearance change could be useful. Not sure if adding this would present any problems, but I wanted to suggest it. :)

Would require adding an inventory to all the blocks (ick). And the safe already has an inventory, so it wouldn't be able to accommodate this kind of functionality. It's a fine idea, just not sure how useful it would be.