I am hoping it is one of the three clocks that we know there are firmware functions for changing frequency.
I would expect the the LClock/MClock/HClock mentioned in older firmwares are system clocks (things like CPU, bus, DSP core). I don't think this would be something that would help you with video sync, my feeling is that these are related to power saving.
ARM946E-S Technical Reference Manual mentions HCLK, which appears to be a bus clock. Don't know if this is related to the HClock mentioned in the firmware.
