I can't imagine installing the inlays that inhabit the Metropolitan Museum's Jacob Kuntz rifle.
I believe that Kuntz engraved much of the inlay BEFORE installing them, only touchup after installation. The matl he used was very thin, I am going to guess around .03 max for the patchbox.
Note all the lines that run OFF the brass, top of eagle's head has a curved line which surely, MY engraving tool would have fallen off the edge of the brass. Numerous examples of that when you look at the leaf edges to the left of the eagle. Another eagle on the far left has a lower beak that extends beyond the engraved line. This tell-tale sign says that not only was the box engraved off the gun, but also engraved BEFORE being cut out from the sheet. Maybe Kuntz was the engraver, and did Rupp's engraving for him, and Rupp stocked Kuntz' guns.
Note the nail that was flush at one time, and then engraved with the design. The lines across the nailhead do not match perfectly what is underneath.
So, yes, I think you CAN engrave and then install, but you'd best use thin material and LOTS of nails to hold the sucker down. You can't file the work flush with the gun, it has to go in in the finished state, after the staining and scrubbing is all done with. I don't believe Kuntz' inlays were ever completely flush with the surface of the gun.
Just ramblin'
Tom