I got this working on Raspberry Pi 3, woo!
I had to modify backend/lib/game/server.js and backend/lib/game/api/auth.js to comment out the greenworks API calls.

backend/lib/game/server.js:

// try {
// if(!greenworks.isSteamRunning()) {
// throw new Error('Steam is not running');
// }
// if(!greenworks.initAPI()) {
// throw new Error('greenworks.initAPI() failure');
// }
// console.log("Connected to local Steam client, using native authentication");
// useNativeAuth = true;
// }
// catch(e) {
// console.log("Connecting to local Steam client failed:", e.message);

// Need these lines to use Steam Web API
 console.log("Using Steam Web API authentication instead");
 if (!process.env.STEAM_KEY) {
 throw new Error('STEAM_KEY environment variable is not set!');
 }
 useNativeAuth = false;
// }

backend/lib/game/api/auth.js:

if(request.body.useNativeAuth) {
 if(!useNativeAuth) {
 return q.reject('authentication method is not supported');
 }
 // Removed greenworks auth stuff... do we need to do something else?
 }

...but now all my rooms are red and I can't choose a spawn... possibly because I can't set a password?