Short Version:
Assign Custom background to an entire Organisation from Teams Admin Portal. Release date unknown, Microsoft: “Working on it”.

Details:
Assign Custom background to an entire Organisation from Teams Admin Portal. This would include the ability to upload a custom Company approved custom Teams meeting background uploaded and optionally set as default background for all users.

Availability:
Unknown. Microsoft: “Working on it”.

Links:
Uservoice request.
Microsoft’s suggested workaround.
Related Tweet.