Decker, congratulations on a fine build, and thanks for posting it here for us to enjoy.
You can definitely engrave inlays right on the rifle, and some inlays will have to be done that way anyway - the wrist oval, the cheek star, etc. But trigger guards, butt plates, lock plates and cocks with their top jaw, and patch boxes can easily be attached to a piece of wood for support either by fitting as in the butt plate, or in Bondo on a scrap of wood, and held in a gravers ball for better control. If you're serious about actually learning how, buy John Schipper's wonderful book - you'll never look back. Apart from the down to earth super instruction and advice he gives, the photo gallery of his and others superlative work will inspire like nothing else can.
Again, nice rifle!