Looks like you beat me to it! I am in the process of recording a bunch of canned responses in Alexa’s voice using the Alexa webapp found here https://alexaweb.herokuapp.com/
Then those recorded mp3’s will be added to a smartapp similar to my Morgan Freeman notification smartapp. While it may not handle custom TTS phrases, I will attempt to cover as many common phrases as possible. I really wanted the smooth, sultry voice of Alexa to tell me when someone is bashing in my front door or the basement is flooding 
I plan to run it using VLCThing + VLC on a RPi