Thanks to @Jenny List for the write-up on the blog, and hello to all the new visitors here.
I will have some progress to report later this week - most of the parts are in for the ARC module. I am a bit worried that it won't work, because I don't understand the HDMI spec enough (I'm not willing to spend $5k to buy a copy). But I've got a cheap commercial ARC audio extractor to sacrifice/copy if needed, so I'll get that damn ARC data somehow.
I've also got an STM32 dev board to start doing some DSP on.
Having second thoughts about the whole backplane concept, it's going to be hard to make power amp modules work (big heatsinks). I may later change to a "hub" board with lots of plugs and some kind of switching logic.