Where to find 478 motherboard circuit schematics?
I wouldn't hold out much hope.  I've looked at motherboards and while they don't look complicated per se the sheer size of the design (number of nets) is bound to be daunting.  Plus I imagine the file format of the schematics will be for a rather high-powered schematic capture program costing several thousands of dollars.  I see you didn't find much for reference designs.  You might have luck finding reference designs for each individual chip (sound codec, south bridge, north bridge, etc) on the board and looking for reference designs.  Ideally a motherboard should just be all of those designs put together, but I'm sure there's several hundred catches (matching impedance for long traces for instance).