As far as I can tell, it’s not possible to have a niente indication centered under a notehead at the end of a diminuendo. I imagine this is because Dorico does not conceive of niente, whether circle or n as a dynamic, but solely as a hairpin modification.
Strictly speaking this makes some sense as niente has to be a final culminating dynamic (how can one continue to hold nothing as a dynamic?) but certainly in terms of visual tidiness, especially in smaller note values where the difference between the beginning of the note and the end of its duration is inconsequential, it would be nice to be able to achieve this. And while there’s a minor semantic difference in what contexts niente is achieved as a dynamic, it still is a dynamic and should behave as such.
I struggle to think of any use case that would suffer if the niente circle or n had the same placement behavior as p or f than its current behavior. In the below example, with current behavior, treating niente as a hairpin modification only allows two positions: immediately preceding a note, or immediately preceding the next note/rest. And if treated as a dynamic, the only change would be adding a middle position of being under the beat.