Those are often called “aviation” connectors (don’t ask me why…)
Some folks succeeded in grounding the spindle themselves internally taking the back off, like in this random image:
My feeling is that things may be slightly more complicated when disassembling a water-cooled spindle (due to the need for the assemble to be water-tight, which may be why they glued it or something). I wonder if it’s worth the risk, maybe you could just ground the spindle body externally and call it a day. Or have fun prying it open.