I'd like to propose some additional options for Render Region.
Right now, it works pretty flawlessly to grab the RR from the LV, but it requires loading the scene which can be time consuming.
Opt 1:
Add a button to let us use the IRR in the c4d window to define the region.
Add a small window that matches the viewport (not LV) and allow dragging in here to define the Render Region.
I'm sure this option would be much more complex, but I figured I'd throw it out there anyway.
Great work as always!
Cheers.