Box2D ANE sounds extremely good. I really look forward to what you will be able to come up with.
(I hope that ANEs are meant to be platform independent.. have not investigated that deep).
The actionscript side is meant to be written in such a way that it does not change depending on your platform, but the native implementation is definitely compiled down to a platform-specific target. For example in this ANE I have to compile a .dll on windows, a .so for Android etc etc, write which library is to be used on which platform in a descriptor and then it all gets compiled neatly into a .ane file, which you can rename to SWC as well.
I had to put this on pause just for a few days because I'm doing a quick contract on the side. I had hoped to have it released by this past weekend but like I said, minor delay. Next target for release is this coming weekend so I'll follow up on that for sure.