There is a protocol involved. It is not complicated and you have access to the source code / documentation for both sides (avrdude, Arduino ISP, bootlader, datasheet). But there is a protocol.
Ok, that's what I feared, thanks. Just wanted to see if this was going to be easy, or wait until I have the patience to dig into a whole big task. Anything that involves the AVR datasheet means setting aside some long hours!
How many are "several Arduinos"?
6-10 to start, maybe more once it's working really well.
P/S Props on your moderator knighthood