I set the work piece onto a block of wood with Bondo. I then clamped that block with the workpiece held fast by the Bondo in my vice and engraved the item. The inlay is flat and I used a piece of wood to make the contour of the cheeck piece. The wood doesn't mar the engraving.
When I inlet the item it isn't perfectly flush, it will stand proud just a tad. That way I don't file into the engraving. I think if I set the inlay into the stock, then engraved it, I would have issues with the convex surface and I would not get the affect I was hoping to achieve. I really don't know any other way to do that.