There's a note field for the trip itself - you could put the actual weight there perhaps? The trouble is, the airline weight machines can be inconsistent - particularly in Asia I've found (Air Asia's weight machines gave a 1.5kg discrepancy when I last took back-to-back flights with them). So to cover those taking multiple flights (which is everyone taking at least a return trip), it would have to be a simple text field anyway to allow multiple values, which is what the note field is. That field (when populated) also shows up in the trip summary right above the bag weights so it's the perfect location for it.