(Wall decor: Engravers? So a gallery, in both art and cavern meanings of the word. But not the theatre/courtroom one.)
I could see it being done with debuilding statues from plinth-positions[1]. Creation would still be at workshops (personally allocatable 'studios'?), but it ties up a need-driven task (like eat/drink/sleep but emotional, perhaps even semi-mood) that involves creation, hauling and (re)placement. Happy and sad thoughts for setting up and unsetting, respectively. Manual (by player!) building of displaced works wouldn't count for 'garden'-related boosts.
Alternately, make it entirely 'engraver'-like. A designated rock cavern area (artificial or otherwise?) could be set like a graffiti-wall, encouraging any who have the need to stop by and use exposed rock (you may have dug it with rock columns, for maximal effect - if multi-Z then another use for stepladders!). Though not sure about over-carving, in that event. But it could certainly give you that sought after Halls Of Moria/Erebor thing.
The big difficulty is that there's a number of functional tweaks needed to have it happen (in whatever form). It might even be just easier to do it without any prior art, like the sudden introduction of temples and libraries.
[1] It would need player/overseer designation of positions within a designated room, at one remove from current working. Being impassible, I don't think I'd trust the dorfs to not wall themselves/others in, requiring additional player intervention/fighting with their artificial stupidity.