I asked Marco's primary NAMM bass demonstrator Matthew Reavly about this a few minutes ago. Here's his take:
Hey guys, I'm new to Talkbass, but I am Matthew Reavley.
I've played one of the fretgrooves that Marco currently has quite a lot, it's his personal one but he does bring it to some of the NAMM shows to demo. I have also ordered one of my own, that is nearing completion at the moment. I also have had a hand in some of the changes that he would like to make to the design and production.
As I had stated in my correspondence with Passinwind, the side of the groove that is closest to the bridge is the edge that is in tune. The opposite edge is not exactly arbitrary though, as the size of the groove lets you feel when you're "in". That said, I only really feel that if I'm pressing in/looking for it. Otherwise, it feels and sounds just like a standard fretless, especially if you have a lighter touch on the left hand. It's worth noting that I am rather experienced with fretless basses as is, and have no trouble playing unlined fretless basses in tune at gigs. I don't feel as though this gets in the way of playing it fretless, but it now allows you as a player more flexibility. I wouldn't tackle chords on a normal fretless, but I would with the fretgroove. Same goes for more tricky runs etc, there's just that bit more confidence in knowing you're in, especially without looking at it.
Also, the photo and the bass is indeed old, he no longer adds the metallic inserts as he found them unnecessary. It does mean you can't use fingerboard materials that are softer, but he generally uses synthetic materials similar to ebonol as they are harder, not sure if he offers this with ebony, but that is probably as soft of a material as would work with this (don't quote me on that).
I've only played them for a few days in total, but I got some good time in with them, enough for me to decide I would like one. Since these basses are quite unique and hard to make, I know Marco is not keen on selling them unless you first get to try them, since they are a fair investment. If anyone is in Singapore and would like to try one, I might be able to do that when I get mine, but I reckon that's not very many of you in this thread.