Those are all good questions to which I don't know the answers. I expect it would be more accurate than amniotic fluid volume estimates because it's a much more straightforward measurement. The studies themselves might give information.

-- Henci 

I don't have anything recent on this issue, but here is the gist of the discussion taken from:

Lieberman E. Risk factors for uterine rupture during a trial of labor after cesarean. Clin Obstet Gynecol 2001;44(3):609-21.

This review is discussing a study (Rozenberg P, Goffinet F, PHillippe HG, et al. Ultrasonographic measurement of lower uterine segment to assess risk of defects of scarred uterus. Lancet 1996;347:281-4.) in which thickness of the uterine scar in late pregnancy was correlated with likelihood of scar rupture. The reviewer notes that using a cut-off of 2.5 mm or less would be a reasonable threshold in that only 8% of the study population had a scar thickness this thin and 10% of them experienced a scar rupture. However, the reviewer also notes that the overall scar rupture rate in the study population was 2.3%. That tells you that the results of this study cannot be generalized to a population that received proper care because any number of VBAC studies have reported scar rupture rates of 0.5% or less.

If you follow the link for Rozenberg to the PubMed site, you will be able to use the "related articles" function to see other, more recent research on this issue. 

-- Henci