The documentation, which needs some improvement, I think.  I'll suggest some improvements when I understand things a little better.

For the record, it turned out that SharedMemoryManager was irrelevant, as were sockets.  That makes sense since memory can't be shared across a network, but the doc nevertheless implies that the socket interface is available.  I don't see why.  ShareableList is a class in shared_memory.py and is a function name, despite its capitalization, in managers.py, with a different signature.  That's massively confusing in combination with the foregoing.  In retrospect, I should have started by paying most of my attention to the documentation's numpy example, even though numpy is irrelevant to my problem and the example was more work to sort through than the other, simpler examples.

With all that resolved in my mind, I immediately ran aground on https://bugs.python.org/issue38119.  In that discussion, Guido notes that [automatic] garbage collection is hard, and I would add that automatic garbage collection is especially hard to deal with when it's not wanted.  I'm attaching the script I wrote in order to satisfy myself that turicas's monkeypatch (see issue38119) allows me to create a standing block of shared memory and to unlink it only when I want to.