It's the worst 3d printer i've ever built and designed. It's the first one, so there's that. A slanted-conveyor-belt-bed dealie. 
I wanted a proper z-probe down to the bed, so I set the duet to CoreXZ and set the conveyor as the Y axis. Blackbelt Cura will do the an angled slices and the whole conveyor-belt thing really well, but it assumes the bed is Z. I uses the post-processor in Cura to swap all the Ys and Zs in the G-code and it worked mostly!
There's definitely some adjustment to do: Z is a little out of wack, leaning hard towards the front of the machine, and there's no on-board print cooling (hence the old case fan in the back). Honestly I was worried I was just going to get spaghetti instead of a kind-of-successful-conveyor-belt-stunt-print (the calicat's hollow inside).
Thanks to the forum members who helped me out by replying to my idiotposting or just helping out others and popping up in searches!