This post caught my attention, and I hope I can help.
When I am not living and breathing food safety and authenticity I do some windsurfing and recently I had to create a perpetual trophy for a windsurfing event. In doing so I had to source some engraving plates to go on the trophy plinth. I ended up purchasing a metal-look engraving plate that is black polypropylene with a shiny silver coating on one side. When it is engraved the writing comes out black. I am not sure if the shiny coating is thick enough to activate a metal detector, but it is a very thin layer so it might not. The material about around 2mm thick and is sold in large sheets that can be cut to size.
I bought the material from an Australian engraving materials supplier but it would certainly have been manufactured overseas so I am sure you could buy it in other countries.
I tried to find a link to the exact product I purchased, but because I ordered it over the phone, I couldn't figure out exactly what I bought. If you search engraving materials you should be able to find something similar. Good luck.