Did Gudo Wafu Nishijima claim that there will be no pain for anyone after death? 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".
First off, the concept of death and rebirth in Soto Zen (Nishijima was a Roshi of the Soto school) differs from the general cosmology during the Buddha's lifetime. 

The Buddha's world-view was greatly influenced by Hinduism, the dominant world view in his region during his lifetime. The original Buddhist concepts of karma, hells and rebirth are basically a facsimile of these concepts in Hinduism and were taken quite literally during and after the Buddha's lifetime.

The Mahayana view on hells and rebirth is not necessarily taken so literally. For example, one hell is populated by constantly angry beings, another by constantly hungry beings with very small mouths, so it's impossible for them to quench their hunger. In several Mahayana schools, it is suggested that humans go through these realms of hell every waking day when feeling angry, hungry, etc. In Nichiren, this is called "Three thousand realms in a single moment of life". You should read Nishijima's statement in this context.

Also, in Mahayana Buddhism, "death" does not necessarily refer to physical death. Reaching a single moment of Kenshō is referred to as "the Great Death".

Lastly, on the concept of "pain". Eastern teachers will often use this word to refer to Dukkha which can be translated as pain in the Pali canon, but is also translated as "uneasy, uncomfortable, unpleasant, difficult, causing sadness". It refers to the unsatisfactoriness of daily life caused by the five Skandhas. This is the cause of traversing "Three thousand realms in a single moment of life".