It looks like you can change the courtesy beep, but you have to be in the program mode to do so.
You -could- build a programmable DTMF board that senses the power loss, injects the audio and does the reprogram to the new tone and subsequently change it back when the power is restored, but that seems like a lot of work (although it would work
I did find the following in the manual (page 3-5):
1. Logic Input #1 Enable
When this channel is enabled, a logic active high input on connector J3-1 will activate voice message #3. This message is user programmable. The default message is "EMERGENCY POWER".
This would be a lot easier. It is unclear how often this message is delivered...the change in the courtesy beep is a more elegant solution.--
Don't ask questions -- just do as you're told.