A couple remarks:
- if st_size == 0, then you have to specify an explicit length (your example passes 0, which would fail with EINVAL without the check)
- most enties in /proc don't support mmap file operation, so it's likely to fail anyway (with ENODEV, EACCESS...). Do you have an example of a /proc entry with st_size == 0 that can be mmapped (mapping /proc/sys/debug/exception-trace fails with EACCESS)?

> How about adding a keyword argument to `mmap.mmap()`, which disables
> fstat-based size checks?

I don't like the idea of adding an argument which doesn't have a counterpart in the POSIX version (especially to address such corner cases).