Right now I think adjusting the CYM-logging code in ym2151.c in mame is my best bet. Right now that logging is just crude register dumps - I don't quite understand how it is supposed to be read and how all the VOPM settings could be interpreted from it. I'd rather make huge, ugly logs that say things like: C1/TL = 0, M1/TL=23, M1/AR=31 ... etc etc.
I am not looking forward to this process by any means - but the cym logging looks totally undocumented ... hence my attempt to ask for help here.
There's really not a lot to say about the CYM logging. It logs each register offset/value byte pair or a '0' byte to mark a 110Hz 'tick' to preserve some timing. It's not really meant to be parsed by hand.
You should stick a load of fprintfs throughout the big ym2151_write_reg switch statement to explicitly log the parameters you're interested in. If you need help with this, let me know. I'm quite a fan of the ol' YM-2151 too