Liam,

We've used the protool guide with 1 1/2" milwaukee self feed bits to hog out mortises before. These bits are like forstners with lead screws. Milwaukee makes a nice bit extension for these in 3 lengths and the one we already had happened to be the perfect length. I think it was 12", but can't remember for sure. They make 2" self feed bits as well.

As far as using the guide, we just took the center ring out like you said and had no problems. With SF bits, there's no point in making a different sized guide ring, as the bit is built basically like a forstner. It didn't seem to torque the frame overmuch. The depth stop alone makes it an improvement over free hand.

I consider it a viable way to produce mortises in a low to medium production environment. And the guide is still usable for peg holes and other tasks with a simple bit change and replacing the center ring.

I'd forget about forstner bits for mortises -- no lead screw makes it way too much work to drill 4 or 5 inches deep.