I will probably take a lot of flak over this, but as you are right now, that is how I would define it. I have a similar relationship with Redpepper. She is married and I am her boyfriend. I am monogamous with her and she is polyamorous. It means your wife is definitely capable of a poly relationship and I would call her polyamorous for sure.
For me being polyamorous hinges on the intimate caring. Because she loves both of you in much the same intimate way that is poly to me. I love your approach and understanding. It is rare to have such "compersion" in a monogamous mindset. I feel it towards Redpepper's husband and know it is real. That makes you special and she should recognize the benefit of how you want her to enjoy this relationship. She is special too in that she has the capacity to love in this manner.
I wish you the best of luck.