Sure you know that D, Paul, it's on the B string at the top of 1/2 position or 3 half steps up from the nut. I know I referenced it kind of oddly referring to the extension, but it would be the second capo past the nut.
I'm thinking the Brandenburgs are going to work better on a 5 string because that has to be accurate and fast, and I know what Ken is talking about with string access. I found this rare portrait of Bach and I think he did it on a different instrument than the one currently used in the Orchestra:
I've tried the little bit of one of the Brandenburgs that I can remember by ear and down low, it's a work-out and it is tough to dig in on the E without skittering on the B or A also. I did put more space between the strings than the bass came with, but if I had it to do over again, I think there is a better spacing strategy than the one I used. I put them on one inch centers, but I should have put one inch between them. This is more difficult to work out at the bridge, but it helps with access to the E. String height is another part of the issue. The B has to be higher than the other strings, so you actually lose a little of the fingerboard arch at the bridge. So having some extra string space would help with that, but I'm thinking a slightly assymetric arch on the fingerboard with a tighter radius on the B side of the board might help also. Some 5-ers have a tighter radius than others, but that is a trade-off also, because you don't want the B string to contribute less tension to the bridge. If it is radiused too tight, the string might not be very strong. I'm thinking the optimum 5 string set up is much harder to get to than the optimum 4 string. Spacing, height and arch all have to be spot on. That said, I think I got mine playing pretty well all things considered. It looks real similar to the set-up on Anselm's bass. The arch looks similar as well. His doesn't seem to have a real tight radius. I'll get a photo of my bridge and post it. It's no work of art, but it works.