I think I was gonna make a post here about my search but I came up without results back then. But FINALLY, I've found an academic source on the Ancient Roman Hobart Mixers.
Technologie und Terminologie der Gewerbe und Künste bei Griechen und Römern, p. 64 lays out a diagram and the actual findings, for the things. Plus a (probably inaccurate but mostly there) reconstruction of the mechanism. A few years ago I got interested in panis quadratum and made a few at an SCA event, baked in dutch ovens, it was fun. But I couldn't re-find the citations, on what I clearly remember seeing and thinking "wait, was that basin with holes in it a kneading machine? holy shit."
It's always been a terrifying, foolish desire of mine, to see just how much fresh bread I can crank out, over the course of a weekend. In this case, quite literally crank. If I could make a portable version of this device out of, say, a barrel or a modern food-safe bucket; I would be one step closer to my fucked up and evil goal of technically-medievally-achievable mechanized bread production.