Basically, it looks like it's not possible. Has something to do with Wight.
even with a cheat or mod?
I'm not sure, but you can try it and let here know if it worked. Once I could get 0 Humanity, but it was caused by a bug. More specifically, it was something with Simon Milligan, he didn't run from that abandoned hospital of Pisha, and I could talk with him and repeat this speak (which should be in the hotel) choosing that "bad" option. I have no idea how to repeat this bug.
This presumably isn't what you want, but typing vstats sell humanity 10
in the console gives you a humanity of 1. I couldn't get it to zero.
The lowest I was able to drop my humanity via natural means was 2.