Star trails - Any solutions for keeping a lens warm?

I have not tried this method myself, but I remember reading a tip on a website from someone in Australia who does a lot of night photography.  He places disposable hand warmers on his  lenses.  You can buy disposable hand or feet warmers in various sizes from any outdoor store such as REI or even Costco this time of year   It will be interesting to see if anyone else has tried this method.  I always carry a couple in my camera bag this time of year in case I need them for my hands, so it would be convenient to have a dual use for them.