I once used a large 'Bell" bit and extension to drill a series of holes through several wall sectioms that I could not otherwise access. I pt the bits in a piece of EMT of similar leagth, just a bit smaller than the diameter of the bit, to keep things straight. The final result wasn't really good enough for running EMT, but was plenty good for easily pulling MC.
That was before I had the impact driver and Speedbor-max bits. For drilling solid wood, that's the way to go. The screw point on the bit feeds it through, and the impact driver does not transmit any torque to your hand - meaning a much straighter hole.
I've used the extensions to extend the reach of the bits as much as 24" - 18" extension + 6" bit - and that was steady enough I expect you could double that if you needed to.
The real key to making any deep hole is clearing your chips. If you have an air compressor, you can make (or buy) a tool to deliver air to the bottom of the hole, and blow the chips out.
