Hope this isn't too late - just noticed this thread.
Our homebrew club purchased a used rye whisky barrel and then filled it with porter. This barrel holds approx 200 liters. One club member has an 80 liter system, and he brewed 2 batches back to back, while I have a 40 liter system and brewed the other bit. We used identical recipes, grain bills, hopping schedules, and yeast. Once the beer was finished fermenting, we transfered it all into the cask. This was last August.
Initially the beer seemed to gain new life from the cask and rapidly got quite delicious. Then after about a month or so, it started to sour. Another club member had about 5 liters of 'swish', which we added to the barrel to try and stop the souring. Swish is made from used whisky barrels. Some water is added to the barrel, which is then resealed, and 'swished' once a day for several weeks. Reasonably strong pseudo-whisky then comes out (approx. 40%+).
At the moment the beer almost defies description. It still has a sour edge, but it is less noticeable now. It has picked up a rye whisky note and there are oak qualities as well. It's quite good, but a drawback is that it is quite thin bodied - not surprising given the infection it picked up. We're going to take it out of the cask in about a month to 6 weeks from now.
If you want that oak character without the infection, try adding steamed oak chips. Steaming them for at least 20 minutes should adequately disinfect them. If they are added in the secondary, the alcohol in the beer will help to inhibit infection as well. If you're keen to experiment, I can heartily recommend adding some quality Scotch to beer too. A good starting point is 1/2 oz in 500 ml. Besides the Scottish Ales, Scotch goes well in old ales, barleywines, porters, and sweet stouts.