I thought about getting a mouthguard for CQB purposes only but instead went with this:
http://www.dedusa.com/products/PROTE...MASKS/DEDMSK01
They designed it for airsoft. It's a thick neoprene-type fabric with a pourous padded mesh in front of the nose and mouth area so you can breathe. The exact materials used is proprietary but whatever it is it works very well.
I've taken extremely close range hits with this directly to the mouth and only felt a slight sting to the lips. It works great.