I have been very curious about this lately. I think it's possible but I'm not too sure. I know it would be really hard to do though. If anyone has made one, please post some pics!
2wd Bike Tech had plans and kits, but went out of business before I could purchase.
Jeep Rubicon Classic Adult All Wheel Drive Mountain Bike.
1991 Legacy Ovation mountain bike.
I'd like to have a flexible shaft coupled to my Staton dual drive gearbox and drive the front wheel for off-road adventures.
Just ordered a spiral bevel set from Ebay. I'll either mount the large bevel gear to the spokes with a custom rag joint or make my own front axle adapter (like a sprocket adapter). Another possibility is to get a 135mm wide fork, a rear free wheel axle and mount the large bevel gear to a right hand thread disc brake adapter.
This is how BWA, Ovation and Christini, attach(ed) the bevel gears on the front wheel:
Wow this is the place to be for discovering new things. I can see some awesome builds which could be built with this technology. I really like the one which let you switch from single to two wheel drive.