Online play between XBox 1 and XBox 360 - can it be done? [Assuming the 360 game is on the XBox1 backward-compatibility list]?

yes. any game that is on xbox 360, and backwards compatible to xbox one, will be able to play multiplayer online, no matter which system you're using.

the xbox one BC feature is mainly a software emulator. so it's running 360 like-software, just through your xbox one. the game servers will be the same, so if you have a xbox 360 and a xbox one, you can play the same online progress on each console.

IF the Xbox One is playing the Xbox 360 version of the game, then YES it can play with other Xbox 360 players.

Otherwise, no.

Yes you can do that with a backwards compatible game.

http://www.xbox.com/en-US/xbox-one/backward-compat...