Cargo metrics in T-100

Payload: Equal to the certificated takeoff weight of an aircraft, less the empty weight, less all justifiable aircraft equipment, and less the operating load (consisting of minimum fuel load, oil, flight crew, steward's supplies, etc).

Freight: On-Flight Market Freight Enplaned (pounds)

Mail: On-Flight Market Mail Enplaned (pounds)

Total Cargo: Freight + Mail

So Payload-Total Cargo, should get you the Total Passenger weight. You could divide that number by onboards to get weight per pax. Not exactly an accurate calculation for all carriers when you factor in carry on bags but it should get you in the ballpark.