I think you have misunderstood how and what is done by what part of your computer.
The cpu doesn't do the playblast, your graphics card does. The reason your not seeing 100% use is because the DG evaluation single threeaded even tough mayas after 2009 are much better threaded. However the fact remains that the playblast is done by your GPU (not CPU) and the limiting factor for speed is your hard drive.
One thing that can help tough is to disable the vertical sync of your gfx card. But yes it will not affect your cpu usage mostly at all.
t's very frustrating to know that my playblast could be faster
Especially because you look ate the worng thing, the cpu isnt the measure here. Really the CPU is just 1/3 of the entire problem for most problems the lowest common denominator sets the limit.