As I want to poke around the aurweb codes and possibly send patches later, I've just fetched the source and dug a little bit.
Turns out, based on how aurweb/initdb.py is written, and digging the internal DB after started aurweb for some testing, there's no field for package arch in DB, so adding an arch key to the DB would be a pretty big and perhaps breaking change (Hardly anyone would want and dare to add a column to an in-production table). But if it really is possible, considering v6 API is pending, there might be enough time window to implement the API change alongside DB change.
It's kinda a shame I don't really have much time until the end of the year to really work on this. May I ask, when will the v6 API be published?
Best regards