Re-checking all possible sources of vacuum leaks, I found the problem tonight. The S-bent flame trap hose had split at both ends, allowing the vacuum line to the intake manifold to have unrestricted flow. If all connections are tight, the vacuum is drawn through the crankcase and balances out the slight positive pressure from blowby, but it's a nearly closed system.

Meanwhile this exercise revealed a bad O2 sensor which will be replaced, but that's secondary. Clearly the knocking at moderate speeds and throttle openings was due to the vacuum leak.