Update: there is now new version here.
It appears you can actually write one in under XX minutes with almost no knowledge of SWF format and complicated parsing process. I have uploaded proof of concept to wonderfl (under unrestrictive WTFPLv2 license :). Processed with its default settings, my away3d SWF looks like this in Sothink decompiler:
Such a code would not even compile, obviously :) To de-obfuscate it, one would have to actually parse SWF, which is still possible, but ends up with meaningless names that are hard for bad programmers to interprete. Feel free to fix bugs or add features.