Last time I had to change my little monkey on a domestic flight*: Baby table down, lav door wide open, and my back/arse hanging out into the galley. The FA actually came over and held the door for me. +1 AA. thank you. (not that the wide bodies have bigger lavs these days...)

As others have said, take just the minimum you need for that one change into the lav with you - theres' no room for a full nappy bag - one diaper, the cream/wipes, and a garbage bag (the reason I still get the occasional plastic shopping bag - to wrap a dirty diaper up in to contain the smells).

*Mrs. Dirty and I have a deal - she puts the food in, I deal with it when it comes out.