Since the mxmlc compiler doesn't yet support this compression, and since it was a PITA to compile Tinic Uro's zlib2lzma on anything but Windows (http://blog.kaourantin.net/?p=124), and since no one else seems to have done it... I wrote a Python script to do the conversion. Woot. It only requires that the input SWF be compiled with
-swf-version=13(or higher). It will fail if you feed it an LZMA-compressed SWF -- feel free to fork me on GitHub and fix that.
python swf2lzma.py in.swf out.swf
Grab the code from: http://github.com/jspiro/swf2lzma