UPDATE 5.05pm Microsoft has just made the announcement official:
ORIGINAL STORY 5.00pm Xbox One now hosts Mass Effect 2 and Mass Effect 3 via backwards compatibility.
Each game's DLC are being added to Xbox.com as we speak.
The first Mass Effect has been available via Xbox One back-compat since the service rolled out.
Neither BioWare or Microsoft has yet made the announcement, but we expect to hear more very shortly. Today is N7 Day, the franchise's annual fan celebration.
More news on Andromeda is also expected.