I've checked out a few of the mob-spawning plugins, but they don't seem to work in peaceful servers. I'm wondering if there could be one which would allow spawning (on command only) of mobs in peaceful servers.
What you would want to do is to turn on mob spawning in the server.properties and turn it off using a plugin like WorldGuard or HeavenlyMob. For WorldGuard, you just type "/region flag __global__ mob-spawning deny".
Great idea, thanks! I think I can do it using Residence, which is the plugin I'm currently looking at for land control.