"success-response":false has nothing to do with whether a guest-agent command is enabled, but rather whether you should expect a response from the agent itself when the command succeeds. For these commands, the answer is no, you do not get a response from the guest agent, because the action performed by the guest has shut things down to the point that it cannot reply; instead, to learn if the command was successful, you must listen to the ordinary QMP monitor for an event notification corresponding to the change in guest state.
Libvirt already handles this interaction correctly.
Ahh, OK. Rats, was hoping I had found a lead to my "no matter what method I try, I can't have a graceful shutdown/suspend of my guest VM's if the host is shutting down/rebooting" problem.