Assuming you are an admin on this server ("I run a server of maybe 20 people") and you have all admin privileges, this is probably the best way to trap the enderdragon without having to lose sight of it (as in the previous answer, where the dragon would be led to an unloaded chunk, thus losing its use as a pet)
Use an inventory-editing software to create a potion of Slowness V that lasts indefinitely. Slowness V will completely immobilize the dragon. Hit the dragon with the potion, then build a glass box around it or something. Protect the glass box with admin perms. Voila, a "a trapped dragon that can be kept safe *maybe in a corner".
Here is a useful link to creating indefinite custom potions, credits to Sethbling.