Hmmm, this sort of sounds like a cascading alert. I realize it's not what you meant exactly, but it is another nice idea (I think). For example, a cascading alert would sound whenever the remaining time is 50% of the elapsed time since the previous alert. So if you set the timer for 20 minutes, it would go off with 10 minutes remaining, then 5 more remaining, then 2.5 minutes remaining... etc. Of course there would have to be a cutoff at, for example, the 30 second mark.
Anyway, that's not exactly what you mean, but I like the idea so I may incorporate it as well.