The Linux host is running the NX server, and I'm able to connect to it using the latest build of OpenNX. But the keyboard mapping is completely messed up. Yes, I tried the workaround suggested elsewhere (editing ~/.nx/config/<host>.nxs, but that appears to do absolutely nothing.
My Mac is recent vintage iMac.
(Also, attempting to resume a session causes OpenNX to crash ... but that's a separate bug).