Because the deep subconscious cannot be experienced consciously, do its ruminations even impinge on the brain to register brainwave activity?

This is not entirely a segregation. The point of differentiation you are asking about is whether there is conscious perception of the presence of the deep subconscious impinging on the brain, not whether there might be some brainwave changes on an EEG that cannot be correlated with actual thoughts, and thus would reflect something is going on in the mind. So we would not categorically rule out the possibility of perceiving an interaction with the deep subconscious showing up in some fashion on an EEG.