I hope at some point we get to dictate something specific to the engravers so we can do things like that on purpose.
Maybe we can get it in a sort of cross between stockpiles and designations? You get a menu of choices similar to the stockpile menu - history, dwarves, artifacts, animals, insects, shapes, etc - and then make the designation. Could probably also do with a "random" option (for when you don't care) and a "custom" option (for when you want something more specific than the broad categories, like a champion killing orcs to engrave his tomb).
Additionally, all levels of engravers can do any sort of image, but the quality of the image at low skill levels makes it hard to tell exactly what was engraved. For instance, at dabbling, an attempt at a historical engraving may result in "This appears to be an image of [stuff], but is little more than scratches. No details are evident." Conversely, a legendary engraver may result in "This is a complex mosaic of [stuff]. The details are minute and exquisite. [Description of details]."
Maybe experienced engravers can also start using any kind of stone you've mined out in their engravings. For instance, to use the above example, "This is a complex mosaic of [stuff], inset with microcline and realgar on a gabbro wall. [yadda]." They wouldn't actually consume any rock boulders in doing this, presumably the inserted stone is little more than small polished rocks, so it can be abstracted a little. Maybe once tools are in you can start polishing stone blanks to be used as insets, cut gems too, and experienced engravers will carry them around to spice up their engravings.