Engaging/disengaging of button operated hand brake should take certain amount of time, just as it is now.
But, if you press and hold the hand brake button, the brake force will be increasing. If you depress it until the full force is applied, hand brake will be disengaged (you can use it in rally cross).
If you hold the button up to full force + eventually hold it more for defined amount of time the hand brake will be locked on, and you will need to press the button again to release.
Of course at the start hand brake should be ON. When it is engaged auto clutch should split the transmission, so you can rev up the engine.
At the green light you can touch the hand brake button, clutch will re-engage the transmission and car will start.