This is killing my in my newest stereo, the bluetooth will sleep if the phone is silent for more than 1 second, like a breathy pause during a audiobook. Then it takes a half second to kick back in. I haven't found anyone with a solution yet.
You would just need to set one of those up, quiet enough not to be too annoying, but loud enough so that the stereo doesn't think its silent.
Also, there are audiobook players (for Android, at least) that skip over silent sections of the audio track.