Many people are road touring on mountain bikes, and the front is the best place to carry gear.
I can't begin to imagine a situation in which you could ever fit enough gear for touring on a front rack alone. For short tours, you ought to be able to fit plenty of gear on a rear rack, unless the bike is a dual-suspension model. If that's the case, or for longer trips, then the best system for touring on a suspended mountain bike is to pull a trailer. You get plenty of load capacity, and eliminate fit problems. It's a single-wheel trailer that uses a special skewer for hitching the trailer to the bike.