Quotes only exist on qtrees (not volumes) and to date, I've always had to refer to LUNs and QTrees by their paths. In this case, since you are setting a quota on a qtree, you have to specify the path to the QT. I imagine the reason why is you can have more than one LUN or qtree in a volume, so specifying the volume name is not specific enough.
I think any API that interacts with either a LUN or Qtree expects a path, not just a name. I'm willing to bet there aren't many (if any) exceptions. Get-NaLun for instance expects a LUN path, not a LUN name. Get-NaQtree is the same way.