It consists of three tools right now:
1. "Armature Symmetry" - A tool that helps making perfectly symmetrical armatures.
2. "Envelope Symmetry" - A tool that helps setting up perfectly symmetrical envelope sets.
3. "Envelope Assignment" - The actual weight assignment tool.
The tool allows defining an unlimited number of envelopes for a mesh/armature rig. Envelopes can have any form as long as they have a convex shape. In fact, simply mesh objects are used as envelopes and configured via their names (with a special syntax).
Every envelope has a bone name and a weight property. Multiple envelopes with different weights can be assigned to each bone.
With executing "Envelope Assignment" vertex groups with respective bone names and vertices are created automatically. Vertices that lie inside an envelope object get assigned to the respective bone with the respective weight (if multiple envelopes apply, the one with the highest weight value gets precedence).
When the rig mesh gets changed at any time, "Envelope Assignment" will update all vertex groups with one mouse click.
I will now demonstrate the current workflow of the 3 tools with some pictures:
"Armature Symmetry"
This is for creating a perfectly symmetrical skeleton. It will parse all bones of one side of an armature and adjust the respective opposite bones. Centered bones are aligned to the center. Opposite bones are created if they do not exist.
In picture 1. a simple skeleton is drawn. Note that only the centered and left side bones are drawn. Also note a) the centered bones are not aligned properly. All left side bones have the suffix "_L"
In picture 2. "Armature Symmetry" gets executed.
Picture 3. is the result of "Armature Symmetry". All left side bones have been doubled, mirrored and given the suffix "_R". Note b) the centered bones have been aligned properly.
In picture 4. c) some bones have been modified.
Picture 5. executes "Armature Symmetry" again.
Picture 6. d) shows the adjusted right side bones.
With this it's really comfortable to create and modify symmetrical armatures.
"Envelope Symmetry"
This works similar to "Armature Symmetry" but it's for envelopes. Only one side of a mirrored envelope has to be created/edited. The opposite site is created/adjusted respectively. The bone and weight assignment works via the name of the envelope objects right now. The syntax is "bone_name:weight". The two lower arm envelopes at 2. a) (below) for example have the following names: "ArmLo_L:1.0" and "ArmLo_L:0.333" corresponding to the bone name "ArmLo_L" and the weights 1.0 and 0.333.
Picture 1. shows a rig with a symmetrical armature.
In picture 2. a buch of envelopes have been created. Note that there's only the left side envelopes.
In picture 3. "Envelope Symmetry" is executed.
Picture 4. shows the result: The right side envelopes have been created and named properly.
In picture 5. b) some envelopes have been changed.
In picture 6. "Envelope Symmetry" is executed again.
Picture 7. c) shows the adjusted right side envelopes.
"Envelope Assignment"
This creates the vertex groups and assigns vertices to them.
Picture 1. shows a model with its envelopes.
In picture 2. "Envelope Assignment" gets executed.
Picture 3. shows the resulting vertex groups of the model mesh.
Picture 4. shows the weights created from the two envelopes for the left upper arm in Vertex Paint Mode.
And finally in picture 5. you see the model with some rotated bones in Pose Mode.
So?
This is the state of my implementation so far. I still have to do some cleanup, configuration stuff and documentation, then I'll release the first version of the script set!
For the future I'm thinkin about maybe adding some weight blending zones so transitions are smoother for higher polycount models.
Any comment/feedback is very much appreciated!
It seems a wonderful tool to assign the weights. 
I've been dreaming about this feature since the beginning of my Blendering!!!!
I'm so damn busy with other work right now, but I'm working on it, it's just going a bit slow... Mainly I'm trying to write some documentation right now.