In the light of issue8485 this looks as a bug. Thank you for investigating this Philip.
I already raised this issue on Python-Dev less than a week ago [1], and only Victor had answered, thus we can assume that the consensus about bytes-like paths is not changed.
The only question is whether we need the deprecation period before removing the support of non-bytes bytes-like paths. Since they are supported during two major releases, and there are even tests for this support, I think that the deprecation period is needed. Third-party code written in last 3.5 years can use this feature.
[1] http://comments.gmane.org/gmane.comp.python.devel/157296