I'm also working throught the details of an upcoming build and have considered the alternatives. Think I'll start with an extinguisher in the cockpit and consider a more automated system as a mod (want to get the car on the road first). Take a look at the coldfire product before you make a decision - it looks like a good choice for our application -
Cold Fire - Multipurpose foaming wetting agent for firefighting - Firefreeze Worldwide. Also click on the "Racing" link on the left side of the page - worth a read.
Regards,
Kevin