The Universal Consciousness into which a Jivanmukta has dissolved would not be confined to brain only. It is all permeating and hence his entire (visible) body should be considered and not the neurons in the brain to find It.
This is a good question and can also be true.
But ever since bilateralism evolved about 600 millions years ago in biological creatures, the body structure of the organism has developed with head and tail ends. The head provided an opportunity to locate a centralized control system for the entire body of the organism. That control structure is the brain. While Consciousness may be distributed all over the body, we may perhaps find more easily identifiable ‘markers’ in the neurons of the brain which is the centralized control system.
Secondly many of such states of the bodily functions which are relegated to invisible and intangible ‘subtler world’ in the ancient Indian tradition (e.g. Brahma viharas) have clear markers in the neurons of the brain. Hence it is reasonable to expect a signature of the Universal Self (into which an individual is dissolved on Realization) in the brain of the Jivanmukta.