So - no success with the 5D.
Or only partial success.
I managed to control the Electro's drawbars via the db-1. However, that only worked for the part in focus. Even by closely and repeatedly following the manual I did not get to a point where the db-1 consistently controlled one part and
only that part, regardless of which one is in focus. Both the digital drawbars on the Electro and the db-1 always ended up controlling the same part.
Now, what I did was: always using the digital drawbars for Part Upper and the db-1 for Part Lower, and switching to the part I wanted to adjust. That allowed me to keep the drawbars and the settings in sync, but I always had to check for the focus beforehand.
I'll play around with the MIDI settings some more, and I have written to Ocean Beach. We'll see!