I just finished reading this, excellent thread!
With regards to the possible uses of irradiating large areas with Royal Blood, I can see this being used in warfare to render unusable sacred or otherwise important sights. As a brief example: Humans attack a small Deep Noble settlement and commit atrocities. The Deep Nobles retaliate by RBing the human's Sacred Mound of High Kingship, meaning that anyone trying to unite the human kingdoms for the next ten thousand years gets radiation sickness.
This brings me to a number of questions I want to ask. Firstly, although they can live pretty much anywhere, the Deep Nobles are essentially cavern dwellers; its their preferred living space. Are they the sole sentient, civilized species to claim dominion over these areas or do they have competition? We know that there is some contact with other species, traders come and so forth and its hinted that the art of writing came to them not solely by force. Are Deep Nobles able to form bonds of association (friendship, professional etc) with non Deep Nobles? How do the different Deep Noble cities feel about each other? Are they all one happy family or are the constant squabbles and alliances like, say, the Greek city states?
As an example, would this situation be possible: A young deep Noble, obsessed by clockwork and mechanical devices lives in a small city, its Royal young and barely the size of a large hill. The place is something of a cultural backwater, mainly evolved in the export of exotic animals found nearby on the surface to larger, far off cities until such a time as its own fields of flesh are self sustaining. As a result, there is no learned master who can teach our young Noble the mysteries of mechanisms. However, it does know that occasionally human traders will bring wonderful clocks and contraptions, seemingly willing to swap them for worthless soft metal. Determined to learn as much about machinery as possible, as quickly as possible to sate the burning passion for a particular chosen subject as only a Deep Noble can feel. The Noble sets out to find the human city and begin their apprenticeship.
I only really thought of this basic situation because I was imagining how much fun you could have with the whole "young Deep Noble on an adventure" thing. Imagine the awkwardness when they set a beggar on fire for being blind and deformed and their confusion when they get chased out of town! "Grr, these humans have no sense of good citizenship at all. Anyway, excellent worldbuilding.