The short answer is this is possible but there is no simple setting that you change and it just works. It takes a lot of effort.
You can't do this directly because ext4 is not a valid format for an initial ramdisk. You must convert the filesystem prior to boot. Mount the Raspbian root partition on another Linux system and then create a CPIO archive of all ...
You should use the command sudo crontab -e and make cron start a bash file that contains your sudo ./fm_transmitter -f 104.0 -r song.wav.
So make a bash file like /home/pi/fm_transmitter -f 104.0 -r song.wav and let crontab run that.
I managed to get Alpine 3.10.2 to boot to the command prompt with HDMI after I copied from a Manjaro Arm for RPI4 install image the following files:
then edited config.txt
No USB keyboard - working on it...