Hi James,
Border Render would be a huge help for me. In my work, using Crowd Render is often pointless since the objects that have to be rendered cover a small part of a very large image (objects inserted in real footage using shadowcatcher), so using Border render allows me to avoid all the empty space which would still take time to render.
If I use crowdrender on 5 equally powerful machines, it still takes a lot longer to render than rendering the small region on a single machine.
I made this camera cropper add-on a while ago which I need to update to 2.8 but would this approach be of any interest to you?
Also maybe I can try and join forces directly in your add-on. I'm not nearly as experienced as you but if you think I can help feel free to ask (I would need pointers on where to look in your code)
You can contact me at chameleonscales@protonmail.com