I tried it several times, there is a dbus message that override blue-tooth rules when phone is receiving a phone-call to put the radio/mp3-player in pause and hear only the phone-call.
But I cannot decode the correct dbus message to emulate this condition when a sms is received to be read by espeak.
Anyway I attach the code of the bash script, it would be run with crone or alarmed.
This system work well if the car-audio-system is permanently tuned to phone audio out.

PS: please be patient if the files contains some errors, I rapidly remove personal data and translated to English from my native language.

Enjoy!!!