Strangely this point was raised over a campfire discussion at CD7 with Geoff Ballard and whilst I don't wear glasses, my best mate does and I have watched him mid race remove the goggles and just ride with his glasses, not good. Anyway Geoff said that he was using the same venting system that my mate had just ordered but not recieved as yet and swore by it. Geoff said that he had done multi hour enduros in some of the worst conditions and never fogged up, something that can be backed up by my freind that has installed it and used it at the last VIPER event in rather shite conditions. Essentially it is a battery driven de-humidifyer and fan venting system from Scott that screws to the underside (inside) the goggle frame. Utilized with the OTG frame system and no fog solution it apperas to be the way to go.
This is not the first time a battery powered unit has been developed (and failed to deliver), but it certainly looks as if a lot of work has gone into it and it actually does what it is meant to do.