DMX Morphs (Blendshapes in Blender) and exploiting a bug in Source that allows bodygroups to share morphs if you edit the outputted DMX files before export. I have a tool to automate the editing, but it only works on the versions of DMX that Game Zombie's tools for 3dsMax export. Blender exports a different version that the tool can't use, and it's non-trivial to add support for that version.