This seems like a very powerful feature, and I would be surprised if they didn't find a solution to the privacy concern - they already added a setting to "show passthrough video when streaming", which is off by default.
I guess that technically everything is there to do it because this is what you have when you configure your guardian.
If it is a matter of privacy or else it should be possible to detect if such a functionnality is used and then prompt to user (at buy time) with an agreement form saying that he/she aggrees for the consequences it might bring. Or find a way to encrypt the input of the camera so only the HMD linked to it can display it.
I understand that some people could missused this functionnality by getting sensible data from the user environment and then use it for analysis and advertisement purposes. But personnaly I would like to have this feature in UE to create an app that wouldn't go on market it self and would be for a private use only.