I have several stories from AO3 that were archived from other sites, the author is always the same account (e.g. ISF_Archivist). But there is the original writer's name, too, only not with an account. Uh, I don't know how to explain.
Here are some links (stories are restricted!):
Four Walls by Twi [archived by ISF_Archivist]
Smoke and Mirrors by Downdilly [archived by ISF_Archivist]
To Snare a Snape by y2kewel [archived by HPFandom_archivist]
Would it be possible to extract the original author's name (like 'Twi') and put it as a second writer in the epub's metadata, so that it displays both in calibre, when collecting the metadata: 'ISF_Archivist & Twi'?