Send string after boot


I build a System with a Raspberry, a battery and a little AVR board to manage things.

What im looking for is a possibility to send a little message over uart after boot (maybe in the moment the startup.wav ist played).
I think this should be done by using rc.local files - but i never used these files. I was looking for the “volumio” startup file, but havent found anything. I also dont know in which rcx.d i should place the startup file.

I also need a signal right after the system halt so i can switch off power (or a signal close to halt so that i switch off 2-3 seconds later). On the other hand it seems that a system halt is signaled when TX pin goes low - so I could detect pin state ("if TX low for more then 1 sec system is halt - switch off power supply)

Another, less important question. Would it be possible to send commands using uart? Like next favorite station, volume, mute,…