Has anyone attempted to repair a leaking harness at the back of the engine? Any success? Outside of a compete harness replacement? 

1. the harness is all one piece. a little seepage out of that hole isn't a big concern. If you wanted to replace the o ring you'd have to remove the valve cover and unplug the injectors and engine brake. If you do go down that road be very careful with the injector connection, the wires get brittle.