Dawnguard COULD contain some form of glitch that could affect anything on the console. You never know.
I remember a while back when something as simple as a patch messed up Xbox Live connection for the Xbox 360. (the patch was for Assassins Creed Brotherhood) Anyone who downloaded this patch lost the ability to connect to Xbox Live. Even if you deleted the patch from your Xbox, you still couldn't connect to Xbox LIve. (until they eventually released another patch a week later to fix this problem, after numerous failed patches that made the matter worse)
Anything is possible.
I don't remember the patch breaking Xbox live for the user, I just remember it breaking the multiplayer component and possible use of the game (Can't play unless you got the title update). If it had broke the Xbox live connection for the player, they wouldn't of been able to have downloaded the fix to correct the issue unless it was uploaded onto a Website which would then be installed via Modio etc.
The whole point of this is that it's a Beta, there are going to be problems and the reward for participating? Receiving Dawguard for free when it's released.