Did you get this working in the end? Need to do this myself at some point so if you find an easy way to get it working please post back here with some tips. thanks.
hi matt no i havent .
The id has to be an int, try:
http://192.168.0.12:8080/jsonrpc?request={"jsonrpc":"2.0","method":"Addons.ExecuteAddon","params":{"addonid":"script.securitycam"},"id":1}}