Yes, I think a monogamous person and a polyamorous person can have a sucessful relationship as you described it, Stead...afterall, anything is *possible*. Having said that, I believe it would take some extrodinary people to have and maintain such a relationship over time.
First, I would expect jealousy to be an issue at some time or another, as it can be in any relationship. However, I think this unique pairing might make the jealousy scale more unbalanced than in other, more "equally yoked", relationships. Not unlike an interacial relationship is in some societies, the mono/poly relationship would face unique hurdles and may feel more societal/familial pressures due to the perceived differences in the involved individuals.
Pair the internal relationship obstacles facing the couple/partners, along with a much greater external pressure than most relationships might have, and I see a recipe for a great challenge facing the couple. At this time, I am only considering the dynamics between the couple and their friends/family. Add in the poly partners and this magnifies the obstacles.
Other mountains to scale would be adding children to the mix. Can only the couple have children together, or can the poly person and partner(s) also have children, or just the poly partners have children? What are the parenting responsibilities of each partner in this relationship? Of course I could go on with the "what if's" and "supposes", but there is no need for that. My point is, though I do believe anything is possible, I also believe such a relationship would face very unique and pronounced challenges...more so than other *like-minded* relationships, where partners are of the same mind in more aspects of the relationship.
I think the key to help any relationship work and keep it strong, is honest communication between all partners, a constant focus on what the relationship goals are (and the wilingness to adjust those goals), plus a keen sense of reality. No relationship is guarenteed to work, no matter how much we work at it. Sometimes life blind sides us and knocks us off balance, out of focus, etc. We can never consider all of the possibilities and cover all of the bases; life is too unpredictable. A mindfullness of that fact actually may help in dealing with life's unexpected events.
Personally, I think mono relationships take a great deal of work, poly relationships take more work due to the fact there are the dynamics of multiple relationships going on and I feel, for myself, that a mono-poly relationship might take the most work of all. Also, I think the the possiblity/probablity for succesful relationships lies with the individuals involved; their mind sets, their life experiences, their abilities to adapt and grow. Even though I think the mono-poly relationship would be the hardest type of relationship for me personally, I think some individuals might just find it the easiest for them! It takes all kinds of people to make this world go round; your question, Stead, gave me something new to consider. Thanks!