Yes, I'm curious about what system this is ... is it a characteristic of the whole system, or a compiler choice to get longer ints?

As to using a Py_hash_t -- it probably wouldn't be as bad as you think.  You might get away with just masking it to throw away the high order bits in dict and set.  (That might not work with perturbation.)  

Even if you have to change it everywhere at the source, then there is some prior art (from when hash was allowed to be a python long), and it is almost certainly limited to methods with "hash" in the name which generate a hash.  (eq/ne on the same objects may use the hash.)  Consumers of hash really are limited to dict and derivatives.  I think dict, set, and defaultdict may be the full list for the default distribution.