I think if you modify a drill guide to accept a larger bit you would not only void the warranty but also put more torque on the frame than it was designed for. The manufacturer or dealer should be able to answer this definitively.
Forstner bits get stuck so often, need frequent clean-out and can send you flying that an auger with twist to clean out the chips is preferable, in my opinion. There a "bit" harder to find but Greenlee and Irwin both make auger bits up to 2", I believe. T-auger or boring machine bits are not designed to be used in electric drills with their higher torque and speeds, so caution there.
Drilling half-size holes with a smaller bit will work but doubles the amount of work and chance for error.