Envelope symmetry?

I have been looking around for some documentation on envelope symmetry but found nothing useful. Is there a magic button for this or a script or something?

I am trying to skin a model that does not under any circumstances want to accept “bone heat” when I bind the mesh to the armature. A weird error message comes up, and I have tried lots of things to solve it without luck. So I decided to go for the envelopes…

Is good ol’ weight painting a better option perhaps?