I like to challenge the view what "correct" behavior is here. If I
pickle an array of 32-bit integer values on one system, and unpickle it
as an array of 64-bit integer values on a different system, is that
correct, or incorrect?

IMO, correct behavior would preserve the width as much as possible. For
integers, this should be straight-forward, as it should be for floats
and doubles (failing to unpickle them if the target system doesn't
support a certain format). For Unicode, I think the array module should
grow platform-independent width, for both 2-byte and 4-byte Unicode.

When pickling, the pickle should always use network byte order;
alternatively, the pickle should contain a byte order marker (presence
of which could also be used as an indication that the new array pickle
format is used). IOW, <i would indicate little-endian four byte
integers, and so on.