0x20 this has been mentioned as a test / experimental / undocumented register for sound functions, no idea how it works.
0x27 contains the set, reset, load flags for timers A and B, which apparently function alongside the YM2612 sound functions, but I'm not sure how these can be used. I believe that this will let you unlock a formant synthesis mode that could in theory be used for speech or complex wave synthesis.
0x2A is the value set for the DAC output for channel 6.
Set the DAC to ON, but don't play samples or waveforms. Instead, send a DC value directly to 0x2A. This will have different effects depending on the hardware you are using, and seems to be most pronounced with model 2 units.
More info here: http://little-scale.blogspot.com.au/200 ional.html
0x2C seems to put the DAC into a "super loud mode", play around with the upper nibble to hear the effect.
More info here: http://forums.sonicretro.org/index.php?showtopic=28589
More info here: http://md.squee.co/wiki/YM2612#Register_.242C