I'm resurrecting this thread to a) update you on how the raisins worked out last year (so nobody makes the same mistake I did) and b) again ask, "How would you have done it?"
So I ended up going with the steaming method. I put the raisins in a hopsack, and steamed them for a while, then threw them in the keg & started a "set and forget" carbonation.
The first pint or so was VERY sweet because the sugars that left the raisins (which had sunk to the bottom) were all hanging around by the diptube
The next bunch of pints were so fantastic. About a full two weeks of delicious, delicious pints, so the raisins had been in there for about 5 weeks now.
One day I came home, had a brew and thought I tasted something... odd that hadn't been present yet.
Next evening, I came home, had a brew, and oh my god it was so awful. All of the raisins oxidized QUICKLY and totally skunked/rotted/nastified the 2nd half of the batch. SO gross and I had to wash quadruple to get the stink out
So yeah... don't use raisins that way. Gonna give this brew another shot though, how would you use raisins in a oatmeal cookie stout?