At high RPMs, the water pump tends to cavitate, an not circulate that much water. Of course at too low of a speed, water does not circulate enough.
There is a single speed at which the coolant is most effective. The advantage of an electric pump would be a single optimal coolant pump speed at any RPM. This is a much greater advantage than saving HP from parasitic loss if you ask me.
I'm sure with a little fab skillz, it wouldn't be too hard. I would use a high quality pump however, like Meziere or something.