Jromers asked two questions: first, is the flavor of citrus pith really umami; second, how could one use citrus pith in a dish.
1. As KYH says, it's not really a question of a flavor being umami. Umami is a quality that a flavor can have. If you want to pinpoint it with some accuracy, grill a steak, being sure to get a very deep crust on it. While it rests, deglaze the pan with a dab of big, mature red wine, add a good deal of sliced shiitake mushrooms and some reconstituted and drained dried porcini, cook until lightly browning, then add the drained and filtered porcini water and a healthy chunk of glace de viande. Put the mushrooms on a hot plate, put the steak on top, pour the liquid sauce over the steak, and then shave good real Parmesan or Pecorino cheese over the top. That's pretty much an umami-fest: it's all savory, all the way down.
That said, no, I simply do not detect this quality in citrus pith to any significant degree. I'd say it's just plain bitter, as many have said, and this is certainly the traditional reason for removing it before using the rind in cooking.
2. Yes, you can use the pith if you want, but you have to be rather cunning about it. The basic thing is that you have to blanch it many, many times. The problem is that the stuff is basically like a sponge, so what you do is to blanch it in a liquid that has some other desirable quality. You will never get rid of the bitterness --- if you want something that has no bitter citrus taste, why are you mucking about with citrus pith? So what you need to do is to mute that bitterness somewhat and balance it against something else you want. The most common thing is to cook whole citrus rind, cut in sticks, in a strong sugar solution. Done thoroughly enough -- and you'll have to change the liquid periodically to remove the bitterness you've infused in it -- this process will mute the bitterness, infuse the rind with a great deal of sweetness, and not lose the basic citrus taste. Then you toss the stuff in sugar to coat and let cool on a silpat or the like. (Don't do this in humid weather: it'll never dry.) When it's dry, eat it as candy, or even better dip 3/4 of each stick in good melted chocolate, then cool and eat.
From this basis, you can infer other possibilities. Suppose you were to infuse the pith with just enough sweetness to balance the bitterness and at the same time add some other flavor to complement the citrus. For example, we know that lime can go very well with a smoky chile flavor, so why not blanch lime rind --- or even pure pith --- in a solution of dark corn syrup or molasses, water, and crumbled chipotle peppers. You'd end up with something you wouldn't want to eat by itself, but if minced and perhaps dried it could make a remarkable accent to something fresh, rich, and a little on the sweet side, say something involving a good deal of avocado. I'm making this all up from my head, of course, but you can think about how one might go about playing with an ingredient of this kind. Myself, I'd be surprised if in a restaurant context you would ever save money by saving pith, but it could be done: just learn to use a very sharp knife to shave the pith from a segment of rind in one clean stroke, and you'll soon have all the pith you could ever want.