I read that all of the downloadable content was included with the Rare Replay backwards compatible 360 titles (Kameo, etc.) Is that true for all games? For example, if I have Red Dead Redemption as a digitally owned "Game on Demand", does it only include the base game? If the downloadable content needs to be purchased separately, is it automatically installed when the Xbox One downloads the image for the 360 title from its servers? And how would it work if the DLC is on disk to be installed (e.g., Borderlands and Skyrim complete editions)?
Yes, the Xbox One backward compatibility feature supports DLC.
Taken from Major Nelson's blog:
Q: Will the DLC for backward compatible titles be available too?
A: Yes. If you own the DLC, you can install and use it just like you do on the Xbox 360. To install DLC you own, go to the Manage Game section for that specific title and install the DLC to your drive.
From personal experience (Fallout 3 GOTY), the on disc DLC was also installed. I believe I needed to put in the 2nd disc to get the DLC to install.
As for digital copies, I would suspect you get the DLC if the digital copy includes the DLC.
As stated above, DLC is supported by backwards compatability.
However, you still need to have access to the actual DLC. Simply playing a 360 game through an Xbox One does not allow you to access the DLC. You need to have digital ownership of the DLC through the Xbox Store, or have the DLC included on the game disc.
Games like Rare Replay and general Game of the Year or Remastered editions all typically come with DLC included with the game. For this reason, you can immediately access the DLC on your Xbox One. You would also have the same DLC access if you were running the game on an Xbox 360.