Can the dharmakaya buddha be manifest as human? It occurred to me that some human life has immeasurable value. Not just is very very valuable, but cannot, in principle, be contrasted with the value of another sentient life.

I think that the term for this immeasurability is the dharmakaya.

Do any traditions claim that the dharmakaya is never manifest in a human being?
I am writing from the point of view of Tibetan Buddhism. Your question actually has two aspects. The only thing that can set humans apart from other sentient beings is realization of "precious human life". This is the desire,ability,and opportunity to take refuge in the Three Jewels of Buddha,Dharma,and Sangha.
The second aspect is the understanding of the three kayas (bodies) of the enlightenment of a buddha.
Dharmakaya represents mind,and as such has no bodily form. It is always present,yet does not change or transform. It is not the conceptual daily mind that we are all familiar with, but rather primordial, pure and clear mind, hence unchanging. It is the basis for determining sambhogakaya and nirmanakaya.
Sambhogakaya represents speech. It is the speech that manifests wisdom in a verbal and nonverbal aspect. In other words, thinking and speaking with wisdom from the Buddhist point of view. It is the ability to communicate wisely on all levels.
Nirmanakaya represents the body. This is the physical aspect of one who is enlightened. It is the literal body that communicates and relates, changing constantly as it purifies karmic influence and residue. This continues until purification to the point of being one with extraordinary ability to work for the benefit of others.
As a sentient being with precious human life,the three kayas work together as one to produce an enlightened buddha.
These topics and many others are covered in Khenchen Konchog Gyaltshen Rinpoche's very easy to understand book,"A Complete Guide To The Buddhist Path".