Are there steel valves available for 16 inch rims? All the ones I have seen are made for much larger, i.e. truck sized tires.
My Workhorse motohome chassis comes with steel jacketed extensions on the rear duals (22.5 inch wheels) and they have performed flawlessly for 3.5 years now. They are held in place by the lug nuts, so they aren't going anywhere. I added aftermarket extensions to the front tires becasue I couldn't monitor or air them without removing the wheel covers.
The only problem I have had with the aftermarket flex extensions is keeping the outside end properly anchored. Some of the aftermarket models use a spring steel mount to fit into a wheel liner opening. They are a one size fits all mount that did not fit two of my rigs very well, allowing the end of the extension to break loose now and then. Once that resulted in damage to the extension and once it cost me a pressure sensor that was screwed onto the extension. Now I have made my own mounts and riveted them to the wheel cover.