Ha, if I'd know I could have brought the bracket of ours back from 'holiday'. It's still on top of the upstairs fridge – no sign of the seat itself though.
But it would certainly have put us overweight for the flight home, so we wouldn't have done anyway.
However, I'm pretty sure we did manage to get panniers on with the Hamax as well – the normal fitting. It's just a case of pushing them on first and accepting they'll be almost inaccessible without taking the seat off. Probably depends on shape of rack etc too.