Sounds like you've already got your first accessory figured out.
You just need to make a simple connector mechanism (an even simpler version of the pickup connectors on the EMG Geezer Butler pickup kit), so that users can hot-swap pickups without having to remove any screws at all.
Heh.....I don't think that's really practical. These percussive pickups involve the whole body structure, including the bridge, and the bridge mount.
But....It does lead to the idea that we may offer different versions of the EUB with different types of pickups, for different styles of players. You guys aren't all the same in what you want from an EUB.
The pickup that failed the test is a further development of the design that I used on the Series IV Scroll Bass. It's made specifically to add a sub-low warble character, like you hear in an upright bass. It worked well on the Series IV, in combination with the quad magnetic pickup. This new version is similar to what I'm planning for the upcoming (someday....) Series V AUB-2. The same principle in how it works, but in a different configuration. I realize now that, that sound character is not like a Baby Bass, or what the Baby Bass guys want.
So, after I get the Baby Bass version sorted out, I may go back and look at offering a version of the bass with the Series IV/Series V pickup. And see if some customers prefer that sound.
Also, there's the quad magnetic pickup, as I used on the Series IV. I still have all the tooling to make them, and I even have a couple of complete units sitting here in a box. How about mounting that, right up near the end of the fingerboard? That could have a really nice sound of its own. Either blended with a percussive bridge pickup, or by itself? Some players may prefer that over the Baby Bass pickup.
These are future options. For the immediate, I need to make the Baby Bass guys happy.
I also realized that I need to invent some new names and definitions for these types of pickups:
Diaphragm Pickup: The Baby Bass and AEB-1 style pickup, where the bridge is mounted on a horizontal thin spring steel sheet diaphragm. The bridge bounces mostly vertical, with some rocking motion. Magnetic coils under the diaphragm detect the movement of the diaphragm. The sound added is a percussive Boom down in the low range, around the bass strings' fundamentals. It's similar to a strike on a Conga at the start of the note.
Percussive Pickup: The style of pickup that I developed for my Series IV AUB-2, and intend to continue on other upcoming models. The bridge is mounted on a spring steel frame, and can move in multiple axes; vertical, horizontal and rocking. It has much more travel than a Diaphragm Pickup, which lowers the frequency of the added percussive sound into the sub-low range. The magnetic coils detect the movement of the bridge in all axes. The purpose is to add a sub-low Pop and Warble to the notes, similar to an acoustic upright bass.
These two types are both Percussive Pickups, but they are tuned differently. The Diaphragm Pickup sounds like tapping on a Conga or Tom; the Percussive Pickup sounds like tapping on the body of an upright bass.