Still testing a new game, but it seems like profile management in Mass Effect Andromeda on Xbox Series X is performed by the Xbox operating system, not the game. So while I can get into the dashboard in a pseudo-"offline" mode, once the game starts up and I assume uses the Xbox API to load a profile and saves... Series X throws the Xbox Live login screen because I'm internet enabled but not authenticated. Nothing I did could bypass this login screen. I couldn't even create a guest profile at that point.

This is only a guess based on taking the network offline, of course. Once I removed internet access, I assume the Xbox OS returned a fully offline profile for Mass Effect to use.