Gudo Wafu Nishijima (incidentally Brad Warner's teacher) the soto monk, claimed that sentient beings in pain are just in hell, and hell is a supposition.
I take this to mean that pain occurs without rebirth, already in this life, andt hat we cannot know that we will suffer pain after death: so I'd conclude that rebirth that isn't painful. What I think I'm adding to his exact words is just that facts like suffering are all or nothing, nothing real is incomplete.
That may seem crazy, but I like it. I'm highly skeptical that we need to experiecne more pain to experience the dharma. Even if sentient beings do (and do not!) experience the result of their evil acts. Is there any basis in the sutras? Not to my conclusion, but his claims about pain. I've read that zen teachers are often ambivalent on their students belief in rebirth, but I mean something more than that, that to experience the result of bad karma is either in this life or not what we usually mean by "painful".