Yeah so you could go two ways with the tank. Since Pask is a bit of an inspirational figure I actually prefer having it not possessed, but rather with large blades featuring impaled Imperial soldiers of different types, perhaps a chained figure or two, and collections of heads lining the barrels of the auxiliary weapons. A tank commander poking out from the top is a great opportunity for a really grisly figure, a sort of Mad Max homage.
On the other hand if you want to go possessed I'd make it a figure of horror - I think having Bloodletters makes it look more comical than anything - and maybe have some removed panels showing red fleshy innards, with protruding blades or perhaps a mouth. A stark yellow eye peering out of red flesh would be good too. I'm a big fan of melded bodies, so faces and hands protruding from the metal, looking as though they've been absorbed by the thing as they've gone under its ram or spiked roller at the front.