Yes i used quite a lot when i played precisions, because i used heavy strings and a high action...so my thumb could slip behind the strings with ease. As you say you develop a sense of muting by touching the strings by raising or lowering the thumb. I use it a little these days on my Artcores, but only behind the E and A, and with my thumb neutral ( more on that later). Due to the archtop of the bass, the low action my thumb can go no further.
As a technique it is sound for the wrist as it has a better angle of approach because the thumb pad sits on the body with the thumb under the strings and the fingers over the strings. It takes out the tendency the wrist has to angle of the body to allow the hand to sit on top of the strings.
I examined and done some studies on the position and found that if strings are the same height as the body the hand ( wrists and finger) work better) To achieve this a bass would need a split level top, in effect from the top half a step down before the E string ( so the neck) and that level continued from there.
But back to the technique in question.
The draw back as i saw it was the to achieve this position for the best, my thumb had to be in line and under the fore finger to create the "slot" that the strings fitted in and allow the ease of muting by lifting the thumb to the underside of the strings. The draw back is that with the thumb under the fore finger, the Carpal Tunnel area of the hand is not flat, the Carpal Tunnel crease is present.
All this means is that the sides of the Carpal Tunnel are being squeezed and that could eventually lead to problems.
When your thumb is to the side, the palm is flat in the Carpal Tunnel area, as you have with a floating thumb technique.
But it may be a pressure issue regarding this point as any pick player will create the same crease in their technique, but the crucial difference is that with a pick it is stable, where as when playing with the fingers the Carpal Tunnel has all the movement of finger picking to deal with ( due to the muscle and ligament movement and attachments in and through the palm) and that squeezing of the Carpal tunnel is a source of friction to the Median nerve within the tunnel...no such issue for a pick player because the pick is held stable and only grip pressure is the problem here.
So by all means develop it and see if it remains stable, but understand the body mechnics behind what you do, and if you develop problems how do you relate it to the technique?...it may be have nothing to do with it, it may be all to do with it, how do you verify what you find or feel?
