I've been recently tinkering with some telephone stuff. There are a number of ways this can be done, but none that I think of are very easy. I've been playing with my own homemade line interface and an MT8870 hooked to a PIC. I can detect rings, on-off hook transitions, remote hang-up on outgoing calls and log all the touch-tones sent/received on the line using this combination. It wouldn't take much for me to add touch-tone generation capability since I have some MT8880s laying around too. :-)
For your application you just need the security device to think that it hears a dial tone. It should then attempt to call out. Since you said that it was dedicated, it shouldn't be all that difficult to ignore the outgoing tones, take the real line off hook, dial the new number, and then connect the phone line to the security box.
Without giving it allot of thought, it seems entirely possible, but it's probably gonna cost you allot to have somebody mock it up unless you can do it all yourself.