Surely unless you are a skilled hacker a password protected dvr with random port is enough to stop the amateur? That is safe enough not to have to scare the customer into saying you are compromising there routers security.
Also i thought that again unless highly skilled the only access an open port gives is to the connected dvr or whatever is connected to that port again with the password it wouldn't even give access to the dvr.
Also the hacker has to know the type of dvr and the port it is using along with the password etc. Type into google hacking port cctv and you see people getting into office systems and supermarkets because they know the type of systems and are skilled/determined for something to do but i can't see them wanting to go through a list of ip's randomly testing for open ports? unless they have software to scan for such weaknesses I'm no expert so this could be the case one day.