Victor Parlati who is a TWC sifu (Brooklyn many years now NY somewhere) posted on here and left before you came I think did Saturday sparring sessions with the face cage helmet and incorporated ground game. He had video of it but I can't find it anymore. Don't know where he's teaching lately but he's a TWC / William Cheung guy so Phil Redmond probably knows.
Ernie Barrios I've also seen do helmet drills - he's a Gary Lam student - www.wingchuncoach.com.
I've done some sparring with them. They have limited visibility but the obvious advantage is full force strikes to the head. It's still a little different than no gear as the strikes to the front of the face don't have penetration damage like getting a bloody nose. Also, a good solid rap anywhere to the helmet at all tends to have a reverberation effect. If you get hit hard you know it right away, and there is a normal adrenaline reaction to that. There was an NFL defenseman Deacon Jones that used to slap helmets so hard he was responsible for the rule change outlawing it, if that gives you an idea. You can spar without gloves with them, but probably should do more open hand strikes against the helmet, except you can punch the face cage.