You can install the Buildcraft mod (client and server) and then the Additional Pipes add-on. The second one contains a block called ChunkLoader, the recipe for it is just
four iron, shapeless.
You can place it in any part of the world, and the chunk it's located in and all four neighboring to it will be loaded all the time, even when there is no one playing on the server. Make sure that the farm is fully covered by pressing F9 and observing blue squares hanging in the air - they indicate chunks, which are forced to load.
However, be careful, don't use too much of them, these limitations there are for a reason.
ChunkLoader recipe may work without
Buildcraft, but I don't know for sure. Try without it first (but you do need to install
MCForge, which are mentioned in the
Buildcraft installation guides).
P.P.S. Alternatively, you could use the ForeverALoad, a mod for the Bukkit.