Ben Finney writes:
The details [of the lockfiles implementing a mutex] are exposed precisely so that they can be customised by programs that need it. This is common, for example, to allow for differing conventions or standards of filesystem layout.
Sure, but the question being asked is why should users of the daemon process library need to worry about that, separately from other users of lockfiles or mutexes? If they need to worry about it only because there's no satisfactory module implementing it widely available, then that could be done in a subsidiary module that handles OS-specific details, and optionally exposes OS-specific special features (a la os and os.posix). That would also make refactoring (close to) trivial when such a module did become widely available.
It's obvious you don't want to go in that direction, which is IMHO good and sufficient reason for you not to do so. But as it stands IMO it's a weakness in the PEP, even if Unix-only is otherwise acceptable for the stdlib.