Putting "incognito" armed marshalls on high-risk flights is another possibility. However, it's expensive, there would probably not be more than one or two of them on any given flight, and it's quite possible that they wouldn't be on the actualy flight being hijacked. In short, it's a "feel good" solution for benefit of public window-dressing, but of little practical value.
"Flight-qualified" CCW sounds like the best solution to me. If the airlines want to encourage pilots and flight attendents to also qualify to carry aboard airlines, so much the better.