Thanks to all for your input - here's the what worked out for me. Taking the idea of a sacrificial piece of plastic I went to the local supplier and found sheets of .020 styrene, which looked almost perfect, especially at about $12 per 4 x 8 foot sheet. I covered the face of the brass, with the protective plastic still on, with transfer tape, then sprayed the back of the tape with 3M Super 77 spray adhesive and stuck the styrene to that. While engraving, the centres all stayed in the letters and when I pulled off the styrene the masking tape came with it and the protective plastic stayed on the brass. The nose rode on the styrene and the chip extractor worked so depth problems and warped material were not an issue. Great ideas, thanks again!
Ian