Having replaced my odometer gears (all three) recently I can tell you it can be done.
Once the plastic "cover" is off the gears you can spin the drive gear (the one by itself) with your finger. This will take forever depending on how many miles you need to add/remove. I would think that a variable speed dremel on a low setting used with a felt buffer tip could acomplish the task quicker.
I wouldn't want to go too fast with it.
Also, since you have the odometer apart, why not go ahead and replace all the gears, it's only ~$60 for the set.
www.odometergears.com