All mesh guards seem to deform on BB hit. Also, shards, with the edges that implies, have been known to come through the mesh and cut. But, they have a nice breathability to them.
Ironfaces seem to work well with some people, especially with a ventilation drilled. They could also be cut down and modified.
This kind of mask
http://www.sportsmans-depot.com/products/Crosman-Tactical-Airsoft{47}Paintball-Face-Mask.html has a flexible guard that allows a weld. The lens suck.
I use a balaclava with a rubber insert over the mouth, and I have been raked on it and have taken hits straight on his from hot guns on it. Neoprene will not work; again, BBs will break teeth through neoprene.
I am not sure if they really will work for airsoft, but a dentist should provide a custom-fit mouth-guard, upper and lower, made of some thin material. He should know if they will provide significant protection. I believe they are rated for heavy sports, so they probably would.
Normal sports mouth-guards are brittle under fire in my experience.