Ever since Sony's brouhaha with their "rootkit copy protection", most audio cd copy protection has been dropped simply because someone will always hack it. EMI and Sony/BMG dropped all mention of copy protection from their cd's since 2006; even Macrovision, makers of Cactus Data Shield, have dropped copy protection technology.
You can create an enhanced audio cd which contains extended data and it somewhat hides the audio tracks from most cdrom drives while cd players can probably play the disc (not sure about older cd players though). This is kind of like a multisession cd but if your burner can read it, so can others.
About the only thing I can suggest is that your clients cut a vinyl album if they want their work copy protected.