It can be done:
2 methods I have used with people to be able to 1 arm chin ups is to do negative only 1 arm chin-ups I.E. start at the top & lower yourself down, you can either use something to stand on to get in postion or use 2 arms to do the concentric contraction and then 1 arm to do the eccentric. You can also start horizontal in the power rack (put the bar acroos the pins ) and work on doing 1 arm chins and raise the bar height as strength increases until you are vertical and can do a 1 arm chin.
You also need continuos neurological stimulation of the motor pathway, a great way to get this is using Pavel Tsatsuline's "grease the groove" method where you set up a chin up bar and every time you pass it you do from 1 - 5 reps (start at 1 and work up to 5) with in a month you can bang out reps easy (works well for any type of chin up / pull up - although the 1 arm is the most difficult and you will need to use the other training tips to get to the 1-2 rep range first) - for a detailed explantion get his book "The naked warrior" - excellent read for learning how to develop extreme levels of muscle tension (tension generates force = greater strength)
Other training tips are to work on grip strength exercises (gripper, farmers walk, deadlift etc.), different width chin-up bars, towel chin-ups. Things to keep in mind, you need to have a favorable strength to weight ratio (fat is just dead weight) and you need high levels of inter-muscular coordination.
The people that do this exercise best are athletes with low body fat who train with either compund free weight exercises, olympic lifts or martial arts / wrestlers. Bodybuilders who have a poor strength to weight ratio (their muscle size is due to mostly: capillary density, increased blood volume, glycogen stores, connective tissue and some contractile proteins) and train in isolation lack the strength and inter-muscular coordination to recruit enough muscles in synchronization to pull their body weight . Last big hands help to get a tighter grip on the bar.
If you have ANY pre-existing shoulder /back/elbow/wrist pathologies DO NOT attempt untill you are completely pain free with a normal R.O.M.
Hope this helps!
S