Well RS I guess it will be some little while before you again seek advice on a topic that is outside the square!
Given the restrictions imposed by the Jag handbrake for your intended purpose, a water spray seems one perfectly logical solution to your problem. It's just that the vast majority of Cobra owners do not indulge in rally or motorcross events, as opposed to straight track work. Have you inquired among the motorcross fraternity? They often have moderately powered cars and may have resorted to the water option if the regs permit.
I'm thinking you need to persist with the Jag rear end either to meet your class requirements or to preserve the authenticity and originality (hate that word!) of your Arntz, but if not you could always put the current Jag unit on the shelf "for later" and instal a Nissan R32 rear end, from the 2.5 turbo GTs, for competition purposes. These are commonly used in Australia. They're stronger and lighter than the Jag and have a fully-functioning handbrake!!