As a follow on to this, he did end up ringing up, although timing ended up still only having a Y->J connection available for booking. As this was not a booking for me, the passenger rang up, and even with all the impossibilities of BA.com never giving straightforward answers about what's available and what it is willing to book (always two different things - and the default answer is almost always Error
Sorry, we cannot currently book the journey you have chosen. You may wish to reselect a different cabin, try again later, or call your service center.), the BAEC center still charged him EUR15 for the phone booking.

Now that a redemption space in J has finally opened up for the ORY-MAD flight, I was going to suggest he call back to see if he can change the Y segment to J (of course we've tried to accomplish this online, and of course it's beyond BA.com's capability).

Does anyone know if he should expect to be charged again to amend the booking this way, and if so, how much?

As another matter, can anyone explain to me why BA.com handles AA redemptions so many times better than it handles IB redemptions. I would have thought that with IAG and all, a BA customer wouldn't be made to feel like he's asking the impossible to redeem with IB a little more seemlessly...