Without taking too far off-topic, would an answer not just be to have proper gate-checking and delivery of bags? Always used to be claimed as an absolute impossibility on BA, but in this day and age when they put those massive tags on buggies and do manage to deliver them at aircraft side at the other end (even in T5!!), why could they not put those same tags on excess cabin baggage that won't fit.

I wouldn't mind (too much) having to give up a bag so long as it was delivered back to me on the jetty at the other end within a couple of minutes of arriving... Quite different to having to risk a one hour wait at the carousel instead.

(Having said this, BA very rudely didn't even allow me enough time to send a text message in the T5 baggage hall the other day before delivering my bag to me, and that was as first pax from the plane thro almost empty passport control. Naturally I was outraged, as not only did that inconvenience me by not being able to complete said text message, but I was also deprived of the righteous indignation I was looking forward to feeling at an outrageously long wait for my bag - well, box actually, but who's counting. Am I entitled to compensation? )