Slightly OT but I had problems yesterday checking in to a BA codeshare (on IB) which would not let me proceed without details of my visa. Problem was for that country, UK Citizens do not need a visa!

Wanted a visa number and expiry date, so got round it by putting in my passport details again in those fields and it was happy. I am sure that would not work for countries like the US which seem to check them in real-time, but might be a work around others can try. IME since your passport is looked at again at the airport, the risk of doing this and having a problem is probably pretty low.