Confirmed, the viscoset ZS56/40/H8 works on the V20C, and since it's under the headset, you get a straight 1/4" lift on your seat and handlebars. I can confirm you can roll it around just by hanging onto the headrest without the front wheel flopping around. I have mine tuned to maximum friction, and it's stiff but smooth. Sadly, it may be another month before I get to properly test it on the road at high speed because it seems like FedEx has lost my Thor seat but won't own up to it. Once I do get my Thor seat, I'll be sure to take it down Mt. Baldy road where I'm typically between 50-60 mph and the road has those damn horizontal cracks that bounce you up into the air mid corner. If I gain any stability or confidence on that DH, I'll consider it a huge win.
For anyone who does install a viscoset, reserve your opinion until you've ridden it 100 miles. Even riding it down the block and back, I can tell it's throwing off my muscle memory for steering inputs, similar to when you change from a 100mm stem to a 60mm stem on an mtb. Most riders will say the 60mm is more twitchy, but in fact, it's just the rider's muscle memory giving too much input. Once you adapt, it just becomes your new normal.
This product requires advanced mechanical skill and special tools to install. If you're running disc brake hoses through the internal routing of the headset, you'll need to cut and install a new barb and olive when you install and everything you want to adjust the tune. Its for this reason I image most are not going to be willing to try more then whatever tune they originally try. If you are dead set on experimenting, I suggest routing your lines outside the frame during your testing faze abd once your set, route everything back inside.