Hi
I think developer like to make multi-room system with more RPi (or other device). Problem is with synchronization. I think it can be done with I2S DAC. I2S protocol use master clock, witch can be use for synchronization. If will be interest I will try to do it.
My board at this moment have own DAC with 4x stereo amplifier, and it can be attach second board so you got extra 4 (stereo) output on one RPi or other device. Board also contain all (unused) RPi ports. So you can attach LCD-s, keys, etc...
My solution is much cheaper than 4 (8) RPi with hifiberry. Quality of sound is the same or better! My system can be use in one big room with up to 16. individual driven! speaker, or you can use it in more rooms (like me). You only need speaker cable to each room. It is good solution for bathroom and outside house, where are a lot of humidity or where is no power or wi-fi signal.