Sonobus looks really promising. It's app is VERY configurable and has clients for every platform, including RPI. I've done an initial test and other than a way-overly-busy UI, it seems to work as described. Obviously, the better you can make your network, the better this will work. QOS and a wired host are important for best performance. But it uses a public server to register the streams and the whole thing is 2-way so could be used for a shout back system too.
I'm quite interested to see what it's like running on a SBC when I have time to test that (won't be for a while) but in the meantime I'm going to continue testing it with computers and mobile devices.