I find this discussion very interesting. A lot of thought has gone into this. While I do not agree with the original poster's sense of need for such protection, I heartily endorse his research into the subject. My BJ8 has new leaf springs and battery cables and the roads I take it on are all in good repair. I have not bottomed out my exhaust yet and i see no reasonable degree of risk that I need to mitigate, but others may certainly have a different situation. Further, if there were a fast, and easy way to install such protection in a fashion that makes the vehicle safer over-all and does not require any irreversible modification, I may adopt it anyway. However, I am concerned that the fuse may, itself, present new grounding points that can cause the very problem we seek to avoid. As stated by others above, I believe the greater threat resides in the trunk itself where metal objects can contact and ground the positive terminal if not adequately protected. Could the solution make this risk worse? For example, now you will have the battery post, plus the connection to the fuse in an area where the hot side could ground to the body. I rely on a tidy trunk with everything well bagged or tied down to avoid that problem.
My lingering question, and forgive me if I missed this in the discussion above, is why we are not considering battery circuit breakers rather than fuses? This is now tried and true technology in the marine world where batteries are called upon to start very large diesel engines, so I don't see the Healey drawing more amps than a Cummins diesel. The circuit breaker gives you a second chance to start the vehicle. Why would anyone want a fuse instead? Perhaps it has to do with the positive ground configuration? I have not researched the sensitivity of the circuit breakers to polarity.