"(though I don’t understand why Apache doesn’t renegotiate while the request body is being sent)"

Apache does attempt to do this, but HttpsConnection is immediately sending the body of the request as fast as the socket will allow, which fills up the SSLRenegBuffer before the renegotiation can occur.

"Or perhaps a new method that explicitly waits for the 100 response."

That is likely a good idea. My httplib.py fork did change the behavior of the endheaders method to return a response object if there was an error returned in response to the "Expect: 100-continue".