Can CPU Speed Stepping be prevented?
It is unlikely you will find a reliable and trivial tool for changing the processor's low level power management away from Apple's recommendations.

As a user, you are limited to the options available in the Energy Saver panel of System Preferences and pmset.

To stop the processor stepping up or down depending on power available, will require a custom kernel extension.

OS X's power management is handled by a layer called IOKit within the kernel. Writing code within the IOKit is not trivial; mistakes can induce kernel panics and do serious harm to the system's stability.

As of 10.8.2, OS X considers 20% battery to be the first stage of a low battery warning.

If your battery is at 5%, it is likely your Mac will shortly be entering emergency sleep or hibernation mode in order to safeguard your data.