Might be more trouble than it's worth, but if you wanted to idle your motor to keep the pump going, then what about this:
You only want the pump going when you're not on the brakes, right?
So, you set it up so that your accelerator comes up not quite all the way, say maybe you have a spring that is not touched when you are driving normally, but when you let off the spring bumps the pedal mechanism to keep it turning at 500 rpm or so.
Now, you have the brake pedal. The brake has an arm that pushes the gas pedal the rest of the way up by pressing on that spring.
When you are at the light, you have your foot on the brake. The light turns green (or the opposing light turns yellow, depending on how you drive) and your foot comes off the brake. As you move your foot to the accelerator pedal, the motor is already spinning the pump, and as long as you're not driving like a teenager your tranny is probably going to be pressurized by the time you start pressing on the accelerator.
IMO, though, the above would not be as simple as adding an external electric oil pump, turned off when the brake lights are off.