To be honest, Carl's example is far superior and cleaner to my one - I really have to put a little more thought into my examples and answers, when I look at the quality stuff all the other people here deliver!
Sorry about the bar check hint. It was only meant for the actual music variables and it was probably more confusing than helpful. All I wanted to say was, that the bar checks in the music still work, even if the time signatures are in a different variable. I assume, this is pretty obivous to most users that already know a little bit about how Lilypond works, but at least to me it was not that clear in the beginning - so I assumed it may help. Looks like the opposite happend - classic example of too much information. :)
All the best,
Christian