While the ability to engrave built walls is a subject of some interest, it's easy to control the quality of the engravings with Dwarf Therapist: simply deactivate the 'engraving' task on all Dwarfs except that single legendary hermit, and designate an area as 'to be engraved'.
So, in my scenario's: a miner digs out an ore vein, going for months without seeing another dwarf, and gets his food from a small stockpile near the entry point to the vein, and sleeps in the bed conveniently provided besides the locked door, which limits his social interaction. When he's done (and I haven't forgotten about him) he gets hauled out of the vein and put in the next.
Next, the whole vein is designated as 'to be smoothed' for a single engraver, who is destined by a higher power (player) to become a legendary engraver. When a throne room is to be engraved, a set of around five novice to average engravers who have been busy smoothing the whole thing, are allowed to stand in awe (but not make eye contact) while the legendary engraver Urist McStinky is taken from his training-vein, and gently pushed into the room, and let loose with the words "Cheese would look good on that wall.".
But if engraving and smoothing are two different tasks, creating legendary engravers would be a whole lot more difficult, and engraving constructed walls and tearing them down would be a labour-intensive training. Also, I've always thought engraving built walls would either require engraving the blocks before building the walls, or changing the blocks to 'engraved' during the engraving of the wall. Deconstructing an engraved wall should not yield smooth blocks.
Alternatively, it could also be possible to un-engrave walls by smoothing them again, provoking the wrath of Urist McStinky who senses his earlier creations being defiled. Who will then serve some time smoothing a jail cell after thinning the herd of apparently expendable smoothers.
EDIT: spelling. Damn this new keyboard.