So be warned - using LV mode shortens life of your cameras. Are you still thinking that underclocking is useless ?
With or without underclocking, the amount of work the CPU has to do in LiveView will be the same. At a higher frequency, it will be in short bursts, with relatively high idle times. At a lower frequency, the CPU will be mostly busy (much shorter idle times). Will this give a significant difference in power draw? I highly doubt.
Is it even possible to adjust the CPU frequency from software, on EOS cameras? Maybe - there are other microcontrollers able to adjust their clock - but here, I have no idea where to look. The closest thing that comes to mind is
this, but it's not what you are looking for.
Reducing the LiveView frame rate, e.g. keeping 10 FPS in photo mode, will make a significant difference in power draw (camera temperature, battery life etc). There is an experimental patch
in the EOS M thread.
Reducing the frequency of the sensor clock, on the other hand, could result in better images (lower noise). No idea where this frequency comes from, and whether it can be configured from software. It has nothing to do with the frequency of the main CPU, though.
Short answer: it might not be useless, but... good luck figuring out how to change it.