As I have asked in response to many such questions. Why?
What application in real life are you planning to enhance by making the dip exercise more challenging? Why not find a movment that engages the extension of the arm in a manner that is safer and has more real world applications? Does your client need to repeatedly climb out of a manhole with a heavy tool belt on? Have you spent anytime evaluating the exercise and comparing it to other movements that could be more appropriate?
I might use dips with a client and seek to increase the challenge for them if they were a competitive gymnast. Otherwise, it would just be a simple dip movement. But I spend a lot of time working post rehab for helping to fix a lot of the damage that is done by exceeding the reasonable manner in which to exercise, so I might just be jaded. Or sincerely trying to follow our code of ethics and be safe and responsible in providing clients with instruction.