Are you sure it isn't the monitor that has an issue? I would check the board's resistors to see if there is an inordinate value on the green before the brown-fix. Maybe the wrong value got used. As much as it would please me to hear that John Carlsen screwed up a third time, best eliminate all other possible causes first.
So far, he has been very responsive to my latest communication. When the troubleshooting is all over, I'll update this thread.
I forgot to update the thread, sorry. In my discussions with the designer, he designed his board by measuring the post-conversion voltage values of his actual Commodore 1084S-D monitor. It's my theory that his monitor was faulty/damaged/non-calibrated when he did his measurements.
I am not intentionally trying to paint him in a negative light, only reporting my experience. I tested two boards, including one certified by him as 100% functional before he sent it to me, and got identical wrong green levels with multiple capture devices and monitors. I even took the entire setup, Gonbes and all, to a vintage computer gathering so that its output could be compared to a real C128 hooked up to a 1084S-D, and no amount of Gonbes POT adjustment could make it look right. It is my personal opinion that the bit-c128 device produces incorrect green output and is not suitable for a CGA replacement.