I'm going to say......probably ok. Reasons: It's not a vertical grip; it has a purpose other than being a grip; it doesn't permit you to hold the gun any differently than if you mounted a large laser or other light under the rail.
Just because it can be used to hold the gun won't make it illegal - the Carbon 15 pistols were designed basically to use the mag-well and magazine as a forward grip.
It might be right on the line, but I don't think it's crossed over into illegal territory.