A machine shop could turn out one of those capstans. From the chainsaw shaft, to the capstan shaft, it would have to be geared down a good bit.
http://www.lewiswinch.com/
Simpson Capstan Winch
Use this with low stretch polyester rope instead of wire cable.. A light pull on the rope will start the winch pulling. Continue taking in rope as the winch does the work. Stop pulling and the winch stops pulling. Capstan style allows for virtually unlimited length of rope as the rope doesn't coil onto the drum. The winch can be anchored away from the load or tethered to the load with the rope secured at destination. Excellent for hunting, four wheeling, pulling logs, loading a boat onto the trailer, or up a steep slope. Used by rescue personnel for it's light weight and dependability. 10 year warranty for the capstan unit. Comes with tethering cable. Line speed varies from 22 to 42 feet per minute depending on load and motor size.
ca0055-sp1.jpg (27019 bytes)
Simpson Capstan Rope Winch is available in two models, either powered by a lightweight Honda gas engine or attaches to most chainsaws. It pulls up to 2,500 lbs (2000 lbs. with Honda motor). It's excellent for pulling big game animals, boats, and for winching stuck vehicles out of extreme locations.
You can pull loads long distances without overheating the motor or experiencing loss of pulling power. Rope is safer than cable if it breaks or snaps back.
The capstan type winch is unique, you're not limited to the length of cable that a drum will hold and the pulling power does not decrease as the drum fills as is the case with drum type winches. You can take up slack in the winch line or play out line, by hand, without running the winch.
Model CS attaches to your chain saw. Saw must be less than 3.5 cubic inch. Needs 3/8" pitch drive sprocket. 2500# capacity depending on size of Chain Saw.
Model SP comes with a 3 cubic inch motor. Can easily mount to a rigid pack frame. Capacity of 2000#.
Specifications and additional information HERE!
http://www.loghomestore.com/tools-chainsaw.shtml?gclid=CKy79KSgjKMCFUlS2goduEPzcA#portable-winches