As far as the missing "D" ring, one may be added (there may be a kit from the manufacturer) but it should be added to the floor, it is certainly not safe to attach ANYTHING to the LBR.
I am wondering how long of a lanyard is your "safety" guy proposing to use?

To answer the 2nd part of your last question, check the operators manual. Do whatever the manufacturer says.
However, I am also pretty sure "the nut buster harness" is not what is required. On a man up order picker, a safety -belt- is acceptable, and in my not all that humble opinion, better, since someone falling off an order picker may not have a 'spotter', like someone in a boom type aerial lift, and may be suffocated or get brain damage as a result of the chest (breathing) restriction that the "nut buster harness" does when someone is suspended by it, before they are noticed.