Hi @Kevin Williams
I see the awesome project is moving along nicely. I have a feature request for a memory mapped 16 bit ISA slot on the ATX board version(or perhaps a single board computer w/ISA between your ITX and raspberry pi board formats), it would seem most useful as your already placing the motherboard in a PC case. I would like it for sound cards, but I think there may be a few other PC cards of use too.
of notable mention for your onboard sound chips, Silicon labs makes a very nice series of PLL clock generator chips in the Si5351 series. they offer 3 to 8 individually configurable outputs between 3KHz to 290Mhz. I mention it as it could be useful if you need to slow the machine down for compatibly reasons with fussy hardware.