[Tristan Lall]

My first thought was the exact same thing that sanddrag suggested.

Also, I'd imagine that you might be able to take the top cover off of a servo and overpack it with grease. Most of them place the potentiometer and motor in the lower compartment, so they would likely be safe enough as long as the grease doesn't flow too much. The viscosity shouldn't be too big a deal, if you use a powerful enough servo (and don't let it get too hot). In any case, once the gears spin a bit, you'll end up with a bit of a cavity surrounding them. The rest of the case should be pretty trivial to seal.