Python: numpy memmap in parallel
Q : "Does it make sense to use numpy's memmap across multiple cores (MPI)?"

Yes ( ... even without MPI, using just Python native { thread- | process-}-based forms of concurrent-processing )

Q : "Can I create a separate memmap-object on each core, and use it to read different slices from the file?"

Yes.

Q : "What about writing to it?"

The same ( sure, if having been opened in write-able mode ... )
Does it make sense to use numpy's memmap across multiple cores (MPI)?
I have a file on disk.
Can I create a separate memmap object on each core, and use it to read different slices from the file?
What about writing to it?