That was me that brought up the transport of nutrients by fungi. And you've got the general idea correct.
One reason that this doesn't work in conventional farming with tillage, is that every time you till, you break up the strands of sub-terranean hyphae. A lot get killed when they are exposed to the sun or dry out. They can't do much transporting until they grow back, and fungi are slow growers unless they have just the right conditions -- then they can grow
really fast.
But once you have a hugelbed established and the transport mechanisms of the fungi are working, yes, you can fertilize here and
some of it will get transported over there. Notice I said "some", because most of the nutrients that you have dropped will be taken up by plant
roots and soil critters in that area. Fungi may be efficient, but they don't get first dibs, and if something else gets to the nutrient first, then oh well.
The other thing to consider is the nutrient in question. Potassium is a very soluble nutrient, unlikely to pool up in one area, so plants and fungi just expect it to be around and have not evolved tranport mechanisms for it. On the other hand, phosphorus is a nutrient that can sit in one area, like a decomposing cow bone, and plants and fungi have to go looking for it. The transport and availability of phosphorus in the soil and how it is assisted by fungi is quite an active area of research.