> but I'm going to have trouble diagnosing things that don't fail
> on my development machine.
On Windows, try any character not encodable into your ANSI code page (eg. Ł with cp1252) in the module path and non-ASCII characters in the module name.
On Mac OS X, sorry, it already works.
On other OSes, set the locale to something else than UTF-8 (and than ASCII because ASCII is not very interesting) and try non-ASCII module names. The patch includes issue3080.py: set the locale to fr_FR.iso88591 to have ISO-8859-1 as locale encoding and try to load a module called 'issue3080\xE4'. U+00E4 is encoded to b'\xE4' in ISO-8859-1 and b'\xC3\xA4' to UTF-8.