Hi Urs,
I have been wanting a way to specify an absolute width for measures since I started using lilypond. It would be great to have this. But I fear it will never be forthcoming. I understand this is difficult if not impossible.
Although I personally think it is a sort of musical abuse of what notation is for and about, the composer I work with insists on drawing scores with 3cm equals one second of time, and I am unable to satisfy this for him. I don't believe notation is a measured time chart, and so I understand why this is not in lilypond. but it would be great in some circumstances, and Urs has a valid use case here.
Andrew