I finally found one of his basses at the local used shop. It was a double cutaway fretless 4-string, with a serial number around 170. The body had been formed out of a conglomerate of leftover wood glued together. This makes for a striking and unique look, but I was dismayed to see some of the blocks coming unglued.
Finish is very rough, but you can order an instrument unfinished and do it yourself. From photos I've seen of people who have taken this route, they look spectacular!
I was initially skeptical of the bassball batlike neck, but found it surprisingly comfortable. I have no doubts about it not needing a truss rod!
Now, about the sound...
The woody sound of the bass itself really comes through, and makes you forget all the imperfections. The approach proves that "less is more", meaning that all the tone variation comes from the players hands and not from "stuff" on the bass.
Wishbasses aren't for everyone, and they quite often are flawed, but they are original, individual, and they sound like nothing I've ever encountered.
So, don't be too critical until you actually try something yourself, and draw informed conclusions.