Scott - the "pony tail palm" is in the family of succulents. It bulbous trunk tissues are a water retention organ, useful in its native habitat.
I agree with Dean & Tim that some kind of critters did your initial damage. You now have some opportunist fungus taking residence. Saturate the suspect area with some sort of broad spectrum fungicide. The important issue is you need to fill the void to eliminate water retention. I would pack your void with a thick mixture of Sacrete. The fungus will not like the lime in it. Plus this would limit the oxygen supply to the fungus. You would essentially be creating an interior rock, displacing the water. You have more than adequate water for this plant's needs. The cambium is alive so there should be no problems if the rotting is corrected or at least arrested.
At least this is what I would do.
Thanks for the feedback! The decay has continued and 50% of trunk is gone now.
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