I think I see my problem.
FS-UAE is expected to send the FSNP string to the game server. NOT the Launcher. The Launcher needs to hit a service (I *just* found service.py) which will spawn a new game server with the appropriate parameters. The Launcher is not able to connect directly to a one-off server instance.
Whew, going to give this a whirl (and revert all the changes I made to fs-uae-game-server-2.2.3.py!) and see how it goes
AgentUSA