Okay. Another thing to consider pertaining to your third point is that top-level tags can live on the iMesh instance rootSet by:

(assuming m is an iMesh instance)
```
m.createTag("cactus")
m.getTagHandle("cactus")[m.rootSet] = some_value
```
Infact, I make use of this in the ```wwinp``` and ```meshtally``` classes. I think at this point we have more than enough places to store data on a PyNE ```Mesh```, so perhaps in the future we should establish some best practices.