As much as I love GB Pox and personally feel it is usually a better choice, I think this deck is better suited as a mono black build. You are not using Green to its potential if you are not heavy into Loam, Crop Rotation, and abrupt decay. I play 27-29 lands depending on the day and I sometime have color issues, make sure it is worth it.
In order to activated the "all players hellbent" clause on Hollow you want to be heavier discard than most Pox. Pox attacks all resources but you want to focus on hands, if you spend a card like sinkhole attacking mana that is more cards stranded in their hand and one less card for you to make them discard with. Liliana Vess fits the deck well setting up Hollow but Sensei's Top can do similar things for less mana, I don't think you are running enough mana for either right now, Top is the most mana hungry cmc 1 card every printed.
If you are really keen on splashing I think Blue may be better, it gives you more ways to set up Hollow with brainstorm or Jace, it also give you Propaganda to stall their creatures without shutting off your Emrakul.
You could also go more combo with a red splash for
Stronghold Gambit, your discard can clear the way, in addition to being great against storm or lands. You can also play Grave Titan for a big threat that can be hard cast in a pinch.
Either way I think you need to commit to a splash if you want to go that way, both getting full value of the extra color and making sure your mana can support it.