Out of interest did you buy the standard 34" scale fretless sets?
Yes, as all my WB's are 34" scale. And surprise, surprise, the FretFx were more accurate than some of my aluminum side dots

Also, the fretless LED's, unlike the fretted version, (I have both) have a light at the first fret. I can't express how helpful this is in playing with proper intonation, at least for a fretless newbie like me. (playing fretted for 32 years, fretless for 3.)
How does the brain unit attach itself to the back side of the headstock? How much weight are we talking about? They do look kool as long as you don't turn the bass over (IMO) ...
The "brain" is nothing more than a battery compartment and a 3 way toggle, hi/low/off. Mostly plastic, a little aluminum, and it takes those 25 cent sized coin batteries. VERY light weight. The battery compartment and the LED array all attach with double sided sticky stuff, no holes, no permanent mods. It's all on the FretFx website. BTW, I have no financial interest in the company, I just love, em. It's true, they are not as inconspicuous as LED's installed in your custom made Alembic, but they aren't an expensive upgrade either. Besides, when the lights are on, no one notices the wire and battery box. Still, if it was just about looking cool I wouldn't bother with them. Did I mention how much they help my playing? Even when there is plenty of light, the LED's are like beacons, leading my errant finger to the right spot.
JBY
P.S. IF you go on the FretFx website, you will see pics of my Scroll in the testimonial section........