Wow Rick, that mounting is exactly what I have envisioned. I've never seen a Bart P Model like those, I wonder how they are wound? Bartolini used to do custom pickups a lot, I've seen some Harp pickup harnesses with one Bartolini pu per string. There is a Lace acoustic bass guitar pickup that works with synthetic cores I believe, but it's in a thin 5-6" soap bar case, designed to slip under the strings on the flat bass top. It has some pretty exotic magnets and potting materials, like the old Lawrence acoustic guitar sound hole models. If any company was to take a chance and was able to mass market an upright magnetic pu that works with synthetic cores, I believe it would be Lace.
Thanks for the photos, that is a lovely EUB. Did you ever find a replacement diaphram pu? I guess those are similar in design to the pu used on the Ampeg Baby bass and the little Supro Pocket bass. Magnetic, not piezo?