Is it OK to use the Air Vent Bath/Dryer Vent on the roof as an exhaust outlet for a direct vent furnace?

In no way should the bath/dryer vent be used for furnace/water heater venting.

In no way should the bath/dryer vent be used for furnace/water heater venting. Furnace vents carry special UL listings and are fabricated in such a way as to not freeze during winter months though exposed to a heat source (think of it as an ice dam over a pipe opening) and are lined so as to not create dangerous situations with contacting shingles. Manufacturers of these vents subject their products to routine testing to retain UL certification for these applications.