You could always use flasm pretty reliably for that, but then you need to edit p-code. Swfmill is still a pretty useful tool for things that can't be done (or done easily) through script. Unless people know what they're doing and are very careful about checking it with something like "flare" to see how it comes out, it's very easy to make the game crash (not because it's modded, but if you make mistakes in things like method call argument numbers and whatnot).

It's not straightforward to get actionscript with classes out because of how mtasc smooshed everything into one package and then had optimized namespace object creation (at least those c*.swf were compiled differently). Not to mention I've only seen ASV by the Manitu Group work without bugs on scripts created with mtasc. (Flare is OK too in order to look at things with.)

I experimented with a script to de-smoosh and unoptimize class creation so they can get sorted out, but even then it'd still be a mess and huge manual process because you have to be careful about merging files since it seems like different swfs were created using different versions of the same classes with things changed here and there.