I just did a quick look at AE's online help in the Auto-Trace section; they're really good about adding comments, which is where you'd find references to a script to do what you want. Nothing there.
Then there's this to consider: unless you've already pulled a good key on these layers, or you are simply Auto-Tracing existing alpha channels, your results would be extremely variable... even if batch Auto-tracing were possible.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
Like Dave said, I can't see this being the best way to do something but a possible work around would be to place all the layers you need to autotrace sequentially > Precompose and then autotrace that one layer.
Tomorrow morning you can just split the results into separate layers.