This question comes up quite often. There are two aspects to it:
1. Coppock was very specific that he had only developed the indicator for the Dow Jones Industrial Index. His work suggested that it captured the outworking of mass sentiment in markets. My own experience is that it works quite well on other similar broad indexes of stock markets, like the S&P 500, the Nasdaq, the FTSE 100, the Nikkei 225 and the Australian All Ordinaries Index. However, it is my experience that pushing it down to sector indexes starts to give poor results. It also does not seem to work as well on some other markets like currencies, bonds etc. As far as individual stocks are concerned, I cannot see why one would even want to use it. There are far better technical analysis tools for trading trends that are far more flexible and robust to be found in any textbook.
2. I have done a lot of testing on the broad indexes I listed above, but I would hardly call it research. I am relying rather on Coppock's idea of mass market sentiment being also evident in those broad indexes. I can also see logically that it might work OK in a very widely traded stock like Telstra, but I have never done any specific work on it, mainly because there are other better tools. I have never seen any research on its use on individual stocks. I would not use it for them unless I had done or seen such research. If you come across this research, please let me know, I would like to see its conclusions and how rigorously it has been done.