Have a look at Fig. 2 here:

http://www.nature.com/nmat/journal/v14/n8/full/nmat4324.html

Metallic waveguides have a range of metamaterial properties, including near 0 effective refractive index and negative effective refractive index.

Hi Edgar,

Let me rephrase your problem to make sure I understand. You already have a MENZ metamaterial and you wish to characterize it. Most likely, you'd like to retrieve its effective properties. To do that you propose to use a waveguide.

If that's indeed what you're trying to do you might find the following reference useful (Open Access):

"Experimental retrieval of the effective parameters of metamaterials based on a waveguide method", Optics Express

Good luck!