Sorry to revive a 3-month old thread, but I'm flying the aforementioned itinerary in a few days and am still hoping to clarify things before I leave. I will certainly get back to you with the result.

Outbound:
YUL-CDG on AF in W (booked in A)
CDG-AMS on KL in Y (booked in A)
AMS-BRU on KL in J (booked in J)

Inbound: in Y (booked in V)

This was all booked in one ticket and I did not intentionally put the AMS-BRU leg in J; this happened automatically in ITA. The ticket was issued by a TA but they could only tell me that the allowance was 2PC, not the specific weight of those pieces. Nor the AF, nor the KL website are of any help because they both specify 23kg for Y/W and 32kg for J without clarification for mixed itineraries.

If anyone can guide me to a written IATA (or any other) rule which I can use to help my case for the outbound journey I would much appreciate it! I will have 4x32kg with me, so it makes a significant difference.

Would it be too simple to argue that I'm allowed 32kg on the J-leg and that the only way I can use that allowance is by receiving it for the complete outbound journey?

Thank you!

Edit for future reference: this case caused a LOT of confusion at check-in. The check-in agent had to dig deep into the airline policy and when she finally found the answer after multiple people had become involved (J allowance for the complete outbound journey), the check-in supervisor denied the J allowance... I was told that the supervisor did not want to admit that she was wrong so she decided to veto against airline policy and refused to honor the J allowance. Apart from having been told that I was right I didn't have a leg to stand on because I did not have the policy in hand. So... I ended up paying for a last-minute upgrade to the front of the cabin because that actually cost less than 4 overweight charges. Money well spent.