My opinion is to not to try to reinvent the wheel on this one. There is a reason every boat that uses a master battery switch is wired on the positive side, with the switch as close to the battery as possible. That reason is you want to be able to disconnect any potential spark source in case a wire comes loose or touches ground. If you do it on the negative side you still have a long run of positive cable just laying there waiting to touch metal and become a dead short.

Now, I know we are talking about a car vs a boat, but same principle applies. If you want something like memory to stay hot all the time, simply connect that accessory in on the positive side of your disconnect side. Everything beyond that is protected when the battery switch is turned off.

Don