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I don't know anything about sign language linguistics, and the Wikipedia page on ASL phonology wasn't very helpful, and suggested wide ranges of allophony.
In considering phonemes as equivalence classes in learning, are there handshape features that are contrastive in some signed languages, but not others?
I've been thinking about this on and off since you posted the question, and the one feature that I can think of I describe below. But it's phonemically contrastive only in certain phonemic contexts.
BSL has a handshape usually called F (this is formed by making a circle with the index finger and thumb). This handshape is usually called 9 in literature about ASL. In ASL a feature which could be called something like "roundedness" (not the same as roundedness in spoken languages!) It's about how round that circle is. So ASL actually has two handshapes, 9 and Flat9. These two are phonemically contrastive, like how English contrasts h and b because hinge is different from binge.
