Grounding a laptop through the DC-jack?
You are over-thinking this.  If you want a handy ground reference to your laptop. you can clip onto the ground/shell of a USB or HDMI (etc.) cable plugged into the laptop. No need to sacrifice a perfectly good cable.

If you really want to make a dedicated cable, use some old USB cable or something easily replaceable.

In reality, as soon as you touch the screwdriver to the screws holding the laptop together, you have ipso-facto "grounded" yourself to the laptop.