Electrical saftey of metal panel-mount DC jacks
This is generally a non-issue. Exposed ground on panel connectors are standard practice. Most metal devices are grounded, using a Chassis Ground. Those metal AV Stereo Systems at home, computer cases, modems and routers, your car, washing machine, dryer, fridge, all have their metal frame and chassis grounded, usually to the same ground on the incoming power cable. Some specialized equipment (mostly studio audio) do not tie their chassis to the common AC ground, but use a Technical Ground, to prevent ground loops or feedback.

If you ever touch (bare) metal on a car, you have touched a ground connected signal with potentially hundreds of amps in it. Death by electrical shock in a car is practically unheard of. Only 1000 deaths a year by electrocution (or subsequent injuries) in the US, and 30% of those deaths are due to High Voltage Live Wire Issues. 10% involve large home appliances. You have more to worry about from your refrigerator than you do from a small DC servo power supply.