I'd like to convert an old 26"-wheel mountain bike that i use as a commuter, to 20" wheels.

My initial thought is to make (or have made for me) a custom fork with a long segment between the fork crown and the lower headset bearing race. So the fork blades would be normal 20" size, then an extension, then the steerer tube. This fork would also adjust the geometry if necessary to compensate for the smaller wheel. Alternatively I could just move the brake posts lower on the existing 26" fork and put a 20" wheel in there, but that would not allow for any geometry adjustments.

For the rear, I'm thinking of an adapter that would clamp into the original dropouts and then have a new set of dropouts 3" below the existing ones. Think of something like an Xtracycle extension but going downward instead of rearword. I'm wondering if I would have major problems with my chain-line. Brakes i could figure out.

Does this seem doable? What geometry changes might i need to make?