Consciousness kasina is in vimuttimagga but is not mentioned in visuddimagga.
There isn’t really a description of how to practice it.

Is it simply investigating infinite/boundless consciousness ?
If so, it’s the only jhana specific kasina.

Is it just mantra work, or something similar?

Or is it a higher path ability where one can solidify their consciousness into a meditative object?

For what its worth the Vimutimagga gives just a bit more than that. From the Vimutimagga pg 130: 

What is the consciousness Kasina? It is the concentration of the sphere of infinite consciousness. This is called the consciousness kasina. The rest is as was taught before.

And the Kasina Sutta:

http://www.buddha-vacana.org/sutta/anguttara/10/an10-025.html