Thanks for the update MrFish
Actually we have used the BB7 semi regularly (when the captain - me - is feeling lazy). Maybe it is the way we have it setup - brake lever on the stokers handlebar, full length cable housing along top tube and down right seat stay. Held on with cable ties.
Our biggest problem is the brake does not always fully release and creates extra "drag" that we don't need
I am fairly certain this is due to the cabling, but no one has any suggestion on a better way to cable this. A number of bike mechanics (who all ride tandems) have strongly suggested to get rid of it.
An option I have looked into might be a Winzip mechanical disk, where both pads actuate (like a Hydraulic brake). However I am hesitant to spend more unless I can get a good solution. Maybe a caliper on the rear might work just as well if I can work out how to mount it.
End of the day, both current rear brakes are only just slightly better than dead weight (except on the 3 occasions where life would have been difficult without the BB7 helping to keep our speed down to a tad over 30kmh on some long windy decents - even then it was a little fast for my liking).