Question on overwriting helper classes

[evad1089]

Is is possible to overwrite vm helper classes? If not, I had a simple idea to make the overwritable.

If all the VM helper classes were surrounded in

if(!class_exists('helperClassName')) {
}

This would be awesome so that system plugins could be written to change the functionality of these helpers without editing a core file. JModel, JView, and JController have that capability so that (perhaps unintentionally), the model, view, and controller classes can be overwritten in this manner.