Bob,
Did you get an answer to this (probably off line)? I'd be interested in something similar. In looking at your requirements, I would think that getting a motion sensor (ActiveEye MS16A) tied to a wireless transceiver (TM751) should do the trick. Set up the ActiveEye (from what I understand, it works better than the EagleEye) to the same code as the transceiver. Then, plug the siren's power supply to the transceiver itself. As soon as the EagleEye detects motion, it would send an "on" signal to the transceiver, which at that point would act as an appliance module and activate whatever it's attached to this outlet.
I currently use an ActiveEye to turn on a flourescent lamp plugged into a transceiver when someone walks into a room. At the same time, through ActiveHome, I set up a timer to turn on/off the same light by sending on/off commands to the transceiver from within ActiveHome.
I haven't tried your scenario, but I hope it works.
Good luck,
Frank