Just a quick note to say that I have made some progress with the Kawai R100 12-bit compression encoding. What Kawai are doing here, near as I can tell, is storing the sound data in logarithmic format. This allows them to perform mixing and volume control in the DGU ASIC using addition, rather than multiplication. Thanks to the fine people on Synth-DIY for mentioning this posssibility.
I hooked the EPROM data lines and the DAC data lines up to a logic analyzer and captured an output (DAC) value for every possible input value. Needless to say it was a lengthy and painful process.
Here is the preliminary result. I have yet to create a proper look-up table (or transfer function), but this is a good start.